Preparation of libraries of protein variants expressed in eukaryotic cells and use for selecting binding molecules

ABSTRACT

The invention relates to methods of producing eukaryotic cell libraries encoding a repertoire of binding molecules (“binders”), wherein the methods use a site-specific nuclease for targeted cleavage of cellular DNA to enhance site-specific integration of binder genes through endogenous cellular repair mechanisms. Populations of eukaryotic cells are produced in which a repertoire of genes encoding binders are integrated into a desired locus in cellular DNA (e.g., a genomic locus) allowing expression of the encoded binding molecule, thereby creating a population of cells expressing different binders.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/308,570, filed Nov. 2, 2016, which is a 35 U.S.C. § 371 applicationof International Application Serial No. PCT/GB2015/051287, filed May 1,2015; which claims the benefit of Great Britain Application Number1407852.1 that was filed on May 2, 2014. The entire content of theapplications referenced above are hereby incorporated by referenceherein.

SEQUENCE LISTING

The instant application contains a Sequence Listing which has beensubmitted in ASCII format via EFS-Web and is hereby incorporated byreference in its entirety. Said ASCII copy, created on Oct. 31, 2016, isnamed 00130_003US1_SL.txt and is 92.9 kilobytes in size.

FIELD OF THE INVENTION

This invention relates to methods of producing eukaryotic (e.g.,mammalian) cell libraries for screening and/or selection of bindingmolecules such as antibodies. Libraries can be used to contain anddisplay a diverse repertoire of binders, allowing binders to be screenedto select one or more binders having a desired property such asspecificity for a target molecule. The invention especially relates tomethods of introducing donor DNA encoding the binders into eukaryoticcells to provide a cell library in which a desired number of donor DNAmolecules are faithfully integrated at a desired locus or loci in thecells.

INTRODUCTION

Protein engineering techniques permit creation of large diversepopulations of related molecules (e.g., antibodies, proteins, peptides)from which individual variants with novel or improved binding orcatalytic properties can be isolated. The ability to construct largepopulations of eukaryotic cells, particularly mammalian cells, whereeach cell expresses an individual antibody, peptide or engineeredprotein would have great value in identifying binders with desiredproperties.

The basic principle of display technology relies on the linkage of abinding molecule to the genetic information encoding that molecule. Thebinding properties of the binding molecule are used to isolate the genewhich encodes it. This same underlying principles applies to all formsof display technology including, bacteriophage display, bacterialdisplay, retroviral display, baculoviral display, ribosome display,yeast display and display on higher eukaryotes such as mammalian cells[1, 2, 3, 4].

Display technology has best been exemplified by display of antibodies onfilamentous bacteriophage (antibody phage display) which over the last24 years has provided important tools for discovery and engineering ofnovel binding molecules including the generation of human therapeuticantibodies. Using phage display antibody molecules are presented on thesurface of filamentous bacteriophage particles by cloning the geneencoding an antibody or antibody fragment in-frame with the geneencoding a phage coat protein. The antibody genes are initially clonedinto E. coli such that each bacterium encodes a single antibody.Generation of bacteriophage from the bacteria using standard methodsresults in the generation of bacteriophage particles displaying anantibody fragment on their surface and encapsulating the encodingantibody gene within the bacteriophage. The collection of bacteria orthe bacteriophage derived from them is referred to as an “antibodylibrary”. Using antibody phage display, antibodies and their associatedgenes can be enriched within the population by exposingantibody-presenting bacteriophage to a target molecule of interest.

To allow recovery of bacteriophage displaying a binder recognising atarget of interest, the target molecule needs to be immobilised onto thesurface of a selection vessel or needs to be recoverable from solutionby secondary reagents, e.g., biotinylated target protein, recovered fromsolution using streptavidin-coated beads. Following incubation of thelibrary of binder-displaying bacteriophage with the target molecule,unbound phage are removed. This involves washing the matrix to which thetarget (and associated bacteriophage) is attached to remove unboundbacteriophage. Bound bacteriophage with their associated antibody genecan be recovered and/or infected into host bacterial cells. Using theapproach outlined above it becomes possible to enrich a subset ofbacteriophage clones capable of binding a target molecule of choice.Phage display libraries have been shown to provide a rich source ofantibody diversity, providing hundreds of unique antibodies to a singletarget [5,6,7].

Historically, display systems for isolating novel antibody bindingspecificities have been based in prokaryotic systems and in particularon display of single chain Fvs (scFv) and to a lesser extent as Fabs onbacteriophage. Display of binders on the surface of bacteria has beendescribed but has not been widely used and applications have largelybeen limited to peptide display or display of antibody fragmentspre-enriched for binders through immunisation [8]. Despite the power ofprokaryotic display systems including phage display there arelimitations. Following selection by phage or ribosome display the genesencoding individual binding molecules are identified by introducing theselected gene population into bacteria, plating the bacterialpopulations, picking colonies, expressing binding molecules into thesupernatant or periplasm and identifying positive clones in bindingassays such as enzyme or fluorescence linked immunosorbent assays(ELISA). Although binding molecules are identified this approach doesnot resolved information on the extent of expression and the bindingaffinity of the resultant clones. Thus although it is possible togenerate potentially thousands of binders, the ability to screen theoutput is limited by the need for colony picking, liquid handling etc.,coupled with limited primary information on relative expression leveland affinity.

Display of binding molecules on the surface of eukaryotic cells has thepotential to overcome some of these problems. In conjunction with flowcytometry, eukaryotic display allows rapid, high throughput selection.It becomes possible to survey millions of cellular clones expressingdifferent binding molecules on their surface. Cell surface display hasbest been exemplified for the display of antibody fragments formatted asscFvs on the surface of yeast cells. A commonly used modality for yeastsurface display makes use of the yeast agglutinin proteins (Aga1p andAga2p). As described by Chao et al. [9], genes encoding a repertoire ofscFvs are genetically fused with the yeast agglutinin Aga2p subunit. TheAga2p subunit then attaches to the Aga1p subunit present in the cellwall via disulphide bonds. Yeast cells expressing a target-specificbinding molecule can be identified by flow cytometry using directly orindirectly labelled target molecule. For example biotinylated target canbe added to cells and binding to the cell surface can be detected withstreptavidin-phycoerythrin. Within a population it becomes possible,using limiting target concentrations, to distinguish those clones whichexpress higher affinity binding molecules since these clones willcapture more target molecules and will therefore exhibit brighterfluorescence. Typically, each yeast cell will display 10,000 to 100,000copies of a single scFv on the surface of the cell. To control forvariation in scFv surface expression in different cells Chao et al useda fluorescently labelled anti-tag antibody to measure antibodyexpression level on the surface of each cell allowing normalisation forvariation in expression level. This approach therefore allows yeastcells displaying high affinity binding molecules to be differentiatedfrom those cells expressing high levels of a lower affinity antibody.Thus using fluorescence activated cell sorting (FACS) it is possible toseparate cell clones according to the affinity and/or expression levelof the encoded binding molecule.

Eukaryotic systems have also proven to be more effective thanprokaryotic systems for the display of multi-chain antibody fragmentsand in particular with larger fragments such as full IgGs, FAbs orfusions of scFv with Fc domains (scFv-Fc fusions). Bead-based or flowsorting-based methods as described above for yeast cells could also beused to select antibodies from display libraries based on highereukaryotes such as mammalian cells. The ability to format displaylibraries and select directly as IgGs, Fabs or as scFv-Fc fusions inmammalian cells would be a further advantage over yeast display. Theglycosylation, expression and secretion machinery of bacterial and yeastcells is different from higher eukaryotes giving rise to antibodies withdifferent post-translational modifications than those produced inmammalian cells. Since the manufacture of antibodies for research,diagnostic and therapeutic application is typically carried out inmammalian cells, display on mammalian cells (or other higher eukaryoticcells such as invertebrate, avian or plant cell lines) could give abetter indication of potential issues or benefits for downstreammanufacturing, e.g., identifying clones with optimal expressionproperties. In addition, antibodies discovered within the context ofdisplay on higher eukaryotes and particularly mammalian cells could beapplied directly into cell-based reporter assays without extensivepurification and without the complicating effect of contaminants frombacteria and yeast cells. Further, libraries of binders could beexpressed directly in eukaryotic reporter cells such as mammalian cellsto identify clones which directly affect cellular phenotype.

Despite the above advantages promised by eukaryotic display libraries,there remain significant problems with creation of libraries of bindersin eukaryotic cells, especially higher eukaryotic cells. Introduction ofa repertoire of exogenous genes (“transgenes”) for expression in highereukaryotes is more difficult than in yeast and bacteria. The cells ofhigher eukaryotes are more difficult to handle and scale up andtransformation efficiencies are lower. Typical library sizes achievedare much smaller. In addition, introduced DNA integrates randomly withinthe genome leading to position effect variegation. Further, donor DNAintroduced into mammalian cells by standard transfection orelectroporation methods integrates as a linear array with variable copynumber of the transfected transgene. The introduction of DNA encoding arepertoire of antibody genes therefore has the potential to introducemultiple antibody genes into each cell resulting in expression ofmultiple distinct antibodies per cell. In addition the presence ofmultiple antibody genes will reduce the relative expression of any givenantibody and will lead to the isolation of many passenger antibody genesreducing the rate of enrichment of specific clones.

Although display of a library of binders on the surface of highereukaryotes is more challenging, some examples have previously beendescribed. In an early publication using mammalian display of IgGsderived from human immunisation, 3 rounds of selection (involvingtransient transfection, cell sorting, DNA recovery and re-transfection)were required to achieve a 450 fold enrichment of antigen-specificcells, averaging 7.6 fold enrichment per round [10]. Similarly transientexpression from immunised libraries expressed withinepisomally-replicating vectors has also been described with antibodiesformatted as scFvs [11, 12] or IgGs [13].

A number of approaches have been described to introduce a single orlimited number of antibody genes into each cell. This includes dilutionof DNA or mixing with carrier DNA [13] but this is a relativelyuncontrolled method for managing copy number of introduced genes andreducing DNA input will have a detrimental effect on library size.Introduction of antibody genes by viral vectors has provided anothersolution to control the introduction of multiple antibody genes percell. A cell surface display library has been generated in this way fromseveral hundred human B lymphocytes generated by immunization andfurther enriched by flow sorting of antigen-specific B cells [14]. Theantibody genes from this enriched pool were formatted as scFvs, clonedinto a Sindbis alphavirus expression system and introduced into BHKcells using a low multiplicity of infection.

Breous-Nystrom et al. [15] used sequential retroviral infection tointroduce a limited repertoire of 91 V kappa antibody genes followed bya heavy chain genes repertoire from 6 healthy donors into a murine pre-Bcell line (1624-5). Infectious retrovirus was generated using the V-Packsystem based on Moloney Murine Leukemia Virus (Stratagene). In order tobias towards single copy insertions, a multiplicity of infection waschose which led to infection of approximately 5% of cells. A majordisadvantage of these approaches is that integration within the genomeis random, leading to potential variation in transcription level basedon the transcriptional activity of the site of integration. Anotherdisadvantage in all these cases is that the integration of the antibodygenes is controlled by limited infection or transfection which impactson library size.

Site-specific integration of transgenes directed by recombinases haspreviously been described. Recombinases are enzymes that catalyseexchange reactions between DNA molecules containing enzyme-specificrecognition sequences. For example Cre recombinase (derived from thesite specific recombination system of E. coli) or Flp recombinase(utilising a recombination system of Saccharomyces cerevisiae) act ontheir specific 34 bp loxP recognition sites and 34 bp Flp RecombinationTarget (FRT) site respectively [16]. Recombinases have mainly been usedin cellular engineering to catalyse site-specific integration. A numberof studies from the work of Chen Zhou [17, 18, U.S. Pat. No. 7,884,054]have described the recombinase-mediated site-specific integration ofantibody genes into the genome of mammalian cells using Flp recombinasewithin the “Flp-In” system,(http://tools.lifetechnologies.com/content/sfs/manuals/flpinsystemman.pdf). The Flp-In system utilises a variety of cell lines which havepreviously had a single FRT site introduced within their genome. Byexpressing the enzyme Flp recombinase it is possible to directintegration of expression plasmids, incorporating a FRT recombinationsite, into this pre-integrated FRT site in target cells.

Using the Flp-In system Zhou et al. [17] introduced an incoming antibodyexpression plasmid containing a FRT site into Chinese Hamster Ovary(CHO) cell line incorporating a FRT site (CHOF cells). Their workdescribes construction of a display library where 4 residues within anexisting anti-OX40 ligand antibody were mutagenised. The library wasscreened using FACS to identify antibodies with anti-ligand affinity onthe cell surface. The overall success in generating improved antibodieswas limited to the isolation of a single improved antibody. The numberof unique mammalian cell clones achieved was not reported.

A follow-on paper by Li et al. in 2012 [18] utilised lymphocytes from ahepatitis B patient to construct an antibody display library. Separatelibraries were produced with the heavy and light chain genes obtainedfrom a donor who had been immunised with HBsAg, individually reported tobe libraries of size 1.02×10⁶ and 1.78×10⁵, respectively. A secondarylibrary was then produced including both the heavy and light chainswhich reportedly had a size of 4.32×10⁵. FACS analysis reportedlyindicated that about 40% of the cells displayed detectable full-lengthantibodies on the cell surface. FACS screening of the library identifiedantibodies binding to HBsAg. Of a sample of 8 selected library memberswhich bound to the antigen, six were found to have the same antibody, soin total three unique anti-HBsAg clones were identified.

The rather limited success of this work may be due to the fact that theFlp-In system is designed for accurate integration in a limited numberof clones rather than large library construction. There is therefore apotential conflict between achieving fidelity of integration versusachieving maximal library size. The Flp-In system utilises a mutant Flprecombinase in the plasmid pOG44 which possesses only 10% of theactivity at 37° C. of the native Flp recombinase [19]. A variant of Flprecombinase (Flpe) with better thermostability and higher activity thanwild type has been identified [19, 20]. This was further improved bycodon optimization to create Flpo encoded within plasmid cCAGGS-Flp_(o)(Genebridges Cat. A203) According to the Flp-In manual however:

-   -   “When generating Flp-In™ expression cell lines, it is important        to remember that you are selecting for a relatively rare        recombination event since you want recombination and integration        of your pcDNA™5/FRT construct to occur only through the FRT site        and for a limited time. In this case, using a highly inefficient        Flp recombinase is beneficial and may decrease the occurrence of        other undesirable recombination events . . . To increase the        likelihood of obtaining single integrants, you will need to        lower the transfection efficiency by limiting the amount of        plasmid DNA that you transfect”        This is echoed by Buchholz et al., 1996 [19]:    -   “FLP may be particularly useful for applications that do not        rely on efficiency but depend on tight regulation”.

In model experiments and using “instructions described in the manual”,Zhou et al. (2010) [17] indeed demonstrated that single copy insertionsoccurred in >90% of clones. In library construction however relativelyhigh amounts of expression plasmid (2.5-3.2 μg per 10⁶ cells) and adonor excess over pOG44 recombinase-encoding plasmid was used [17, 18].The Flp-In system recommends using a ratio of at least 9:1 in favour ofthe recombinase encoding plasmid versus the expression plasmid. However,when seeking to increase library size by transfecting larger amounts ofDNA there is the potential for random integration of the incomingplasmid [21]. In all studies the accuracy of integration and the numberof integrants per cell under “library construction” conditions was notreported.

In nuclease-directed integration of genes a site-specific nuclease isused to cleave cellular DNA at a specific location. It has previouslybeen shown that this enhances the rate of homologous recombination by atleast 40,000 fold and also allows repair by non-homologous end-joiningmechanisms. This enhancement of site-specific integration has notpreviously been used or contemplated to solve the problems associatedwith creating libraries of binders.

US20100212035 describes methods for generation of rodents capable ofexpressing exogenous antibody by targeting the immunoglobulin locus of amammalian embryo with a meganuclease to direct integration of a donorDNA. The potential to create variant libraries of meganucleases tocreate new DNA cleavage specificities is described but it his does notcontemplate the use of meganucleases towards the generation of librariesof binders.

WO 2013/190032 A1 describes integration of genes into a specific locus(Fer1L4) previously modified with exogenous DNA (“a site specificintegration” SSI host cell) to incorporate recombinase sites, such asloxP and FRT sites for recombinase-mediated site-specific geneintroduction. Nuclease-directed library generation is not described.

WO 2012/167192 A2 describes targetting genes to a locus that can then beselected for amplification. Nuclease-directed methods are employed totarget the locus. Nuclease-directed library generation is not described.

US 2009/0263900A1 describes DNA molecules comprising homology arms andtheir use in methods of homologous recombination. Nuclease-directedlibrary generation is not described.

WO 2011/100058 describes methods for integration of nucleic acid into agenome that avoids the need for long homology arms and instead relies onmicrohomology or “sticky ends” on the genome and donor to help directintegration. Nuclease-directed library generation is not described.

WO 2011/090804 describes methods for integration of multiple genes ormultiple copies of the same gene using different zinc finger nucleases(ZFNs) in sequential rounds. Nuclease-directed library generation is notdescribed.

WO2014/039872 describes methods for engineering plant cells,incorporating a “landing site” into which donor DNA is integrated byhomologous recombination or non-homologous end joining usingsite-directed nucleases. Bacterial artificial chromosome (BAC) librariesare used for initial cloning of donor DNA. Libraries are mentioned inrelation to Illumina sequencing methods. Nuclease-directed librarygeneration is not described.

WO2007/047859 A2 describes methods for engineering specificity ofmeganucleases and their used to target genomic loci. Libraries of mutantmeganucleases that may contain meganucleases with new nucleasespecificity are described. Nuclease-directed library generation is notdescribed.

US2014/0113375 A1 describes a transient expression system for generationsingle-stranded DNA sequences homologous to a target genomic sequence,which can be transported to the nucleus to alter the genetic informationof the target genomic sequence via DNA repair pathways or homologousrecombination. It is suggested that a “library” of mutations could becreated by low fidelity reverse transcription of the introduced(non-library) DNA. Mammalian display and selection of molecules withbinding activity is not described.

US2012/0277120 describes methods and compositions for the simultaneousintegration of a plurality of exogenous nucleic acids is in a singletransformation reaction using the native homologous recombinationmachinery in yeast, which recombination may be further enhanced byinducing targeted double-strand breaks in the host cell's genome at theintended sites of integration. The methods are intended to overcome theneed for multiple rounds of engineering to integrate multiple DNAassemblies, for example, for the construction of functional metabolicpathways in industrial microbes, such as yeast. The display orexpression of libraries of binding molecules, the use of highereukaryotes and the selection of molecules with binding activity is notdescribed.

To fully realize the potential for antibody display on mammalian cellsand other higher eukaryotes there is a need for a system to create largelibraries which combine accurate integration into a pre-defined sitewith an efficiency that allows construction of large libraries.

SUMMARY OF THE INVENTION

We have overcome the problem of creating large libraries of bindersencompassing one or two binder genes per cell by using nuclease-directedintegration of populations of genes encoding binders. The invention thusallows preparation of populations of eukaryotic cells wherein arepertoire of binder-encoding is integrated into a fixed locus in thegenome allowing expression of the encoded binding molecule, therebycreating a population of cells expressing different binders.

The present invention relates to methods of producing eukaryotic celllibraries encoding a repertoire of binding molecules (“binders”),wherein the methods use a site-specific nuclease for targeted cleavageof cellular DNA to enhance site-specific integration of binder genesthrough endogenous cellular repair mechanisms. Site-specific nucleasespermit the accurate introduction of donor DNA encoding binder moleculesinto one or more defined loci within the eukaryotic genome or othereukaryotic cell DNA. The invention provides methods of preparingpopulations of eukaryotic cells in which a repertoire of genes encodingbinders are integrated into a desired locus in cellular DNA (e.g., agenomic locus) allowing expression of the encoded binding molecule,thereby creating a population of cells expressing different binders.

Construction of libraries of binders within eukaryotic cells accordingto the present invention has advantages over recombinase-directedapproaches for site-directed incorporation of expression constructs. Thepresent invention uses cellular DNA cleavage by site-specific nucleasesto solve problems previously associated with construction of largerepertoires of binder genes in eukaryotic cells and particularly highereukaryotes. This invention allows the efficient creation of largepopulations of cell clones each expressing individual binders integratedat a fixed locus in cellular DNA. From these libraries of cellularclones it becomes possible to isolate genes encoding novel binding orfunction-modifying proteins and peptides.

Rather than recombinase-directed exchange of DNA, the approach of thepresent invention utilises site-specific cleavage of cellular (e.g.,genomic) DNA followed by the use of natural repair mechanisms tointegrate binder-encoding donor DNA. Following cleavage of the cellularDNA at a sequence recognised by the site-specific nuclease (“recognitionsequence”), breaks in the cellular DNA are repaired using mechanismssuch as homologous recombination or non-homologous end joining (NHEJ).Creation of site-specific breaks in the cellular DNA enhancesincorporation of exogenous donor DNA allowing the construction of largepopulations of cells with binder genes integrated at a fixed locus.

To date, site-specific nucleases such as meganucleases, ZFNs, TALEnucleases and CRISPR/Cas systems have been directed towards theefficient creation of cells with modifications to endogenous genes orfor introduction of reporter genes for the study of cell function. Thereare also instances where nuclease-directed genomic targeting has beenused to integrate genes encoding single secreted antibodies for antibodyproduction (by purification from culture medium) [21, 22,].

The invention simplifies construction of large libraries while directingintegration to a single or limited number of defined genetic loci.Integration of donor DNA at one or more fixed loci normalisestranscription compared with random integration of variable numbers oftransgenes, and allows selection of antibody clones on the basis oftranslational and stability properties of the binder itself. Faithfulintegration of donor DNA at a pre-determined location or locations inthe cellular DNA results in relatively uniform levels of transcriptionof binders in the library, and high efficiency of donor DNAintroduction, make cell populations created by the methods of theinvention particularly useful as libraries for display and selection ofbinders. Methods of the invention thus produce high quality libraries ofbinders in eukaryotic cells, which can be screened to identify cellsencoding and expressing a specific binder for a target of interest.

In various aspects the invention relates to new and improved methods ofpreparing eukaryotic cell libraries, the libraries themselves, isolationof desired binders, encoding nucleic acid and cells from the libraries,and uses of the libraries such as for expression and screening ofbinding molecules and for screening for the effects of bindingmolecules. Various methods will be described for producing libraries invitro and using libraries in vitro or in vivo.

The invention provides a method of producing a library of eukaryoticcell clones containing DNA encoding a diverse repertoire of binders, themethod comprising using a site-specific nuclease to target cleavage ofeukaryotic cell DNA to enhance site-specific integration of binder genesinto the cellular DNA through endogenous cellular DNA repair mechanisms.

A method of producing a library of eukaryotic cell clones containing DNAencoding a diverse repertoire of binders may comprise:

providing donor DNA molecules encoding the binders, and eukaryoticcells,

introducing the donor DNA into the cells and providing a site-specificnuclease within the cells, wherein the nuclease cleaves cellular DNA tocreate an integration site at which the donor DNA becomes integratedinto the cellular DNA, integration occurring through DNA repairmechanisms endogenous to the cells.

For multimeric binders comprising at least a first and second subunit(i.e., separate polypeptide chains, such as antibody VH and VL domainspresented within a Fab or IgG format), the multiple subunits may beencoded on the same molecule of donor DNA. However, it may be desirableto integrate the different subunits into separate loci, in which casethe subunits can be provided on separate donor DNA molecules. Thesecould be integrated within the same cycle of nuclease-directedintegration or they may be integrated sequentially usingnuclease-directed integration for one or both integration steps.

Methods of producing libraries of eukaryotic cell clones encodingmultimeric binders may comprise:

providing eukaryotic cells containing DNA encoding the first subunit,and providing donor DNA molecules encoding the second binder subunit,

introducing the donor DNA into the cells and providing a site-specificnuclease within the cells, wherein the nuclease cleaves a recognitionsequence in cellular DNA to create an integration site at which thedonor DNA becomes integrated into the cellular DNA, integrationoccurring through DNA repair mechanisms endogenous to the cells, therebycreating recombinant cells which contain donor DNA integrated in thecellular DNA. These recombinant cells will contain DNA encoding thefirst and second subunits of the multimeric binder, and may be culturedto express both subunits. Multimeric binders are obtained by expressionand assembly of the separately encoded subunits.

In the above example, nuclease-directed integration is used to integrateDNA encoding a second subunit into cells already containing DNA encodinga first subunit. The first subunit could be previously introduced usingthe techniques of the present invention or any other suitable DNAintegration method. An alternative approach is to use nuclease-directedintegration in a first cycle of introducing donor DNA, to integrate afirst subunit, followed by introducing the second subunit either by thesame approach or any other suitable method. If the nuclease-directedapproach is used in multiple cycles of integration, differentsite-specific nucleases may optionally be used to drivenuclease-directed donor DNA integration at different recognition sites.A method of generating the library may comprise:

providing first donor DNA molecules encoding the first subunit, andproviding eukaryotic cells,

introducing the first donor DNA into the cells and providing asite-specific nuclease within the cells, wherein the nuclease cleaves arecognition sequence in cellular DNA to create an integration site atwhich the donor DNA becomes integrated into the cellular DNA,integration occurring through DNA repair mechanisms endogenous to thecells, thereby creating a first set of recombinant cells containingfirst donor DNA integrated in the cellular DNA,

culturing the first set of recombinant cells to produce a first set ofclones containing DNA encoding the first subunit,

introducing second donor DNA molecules encoding the second subunit intocells of the first set of clones, wherein the second donor DNA isintegrated into cellular DNA of the first set of clones, therebycreating a second set of recombinant cells containing first and seconddonor DNA integrated into the cellular DNA, and

culturing the second set of recombinant cells to produce a second set ofclones, these clones containing DNA encoding the first and secondsubunits of the multimeric binder,

thereby providing a library of eukaryotic cell clones containing donorDNA encoding the repertoire of multimeric binders.

Site-specific integration of donor DNA into cellular DNA createsrecombinant cells, which can be cultured to produce clones. Individualrecombinant cells into which the donor DNA has been integrated are thusreplicated to generate clonal populations of cells—“clones”—each clonebeing derived from one original recombinant cell. Thus, the methodgenerates a number of clones corresponding to the number of cells intowhich the donor DNA was successfully integrated. The collection ofclones form a library encoding the repertoire of binders (or, at anintermediate stage where binder subunits are integrated in separaterounds, the clones may encode a set of binder subunits). Methods of theinvention can thus provide a library of eukaryotic cell clonescontaining donor DNA encoding the repertoire of binders.

Methods of the invention can generate libraries of clones containingdonor DNA integrated at a fixed locus, or at multiple fixed loci, in thecellular DNA. By “fixed” it is meant that the locus is the same betweencells. Cells used for creation of the library may therefore contain anuclease recognition sequence at a fixed locus, representing a universallanding site in the cellular DNA at which the donor DNA can integrate.The recognition sequence for the site-specific nuclease may be presentat one or more than one position in the cellular DNA.

Libraries produced according to the present invention may be employed ina variety of ways. A library may be cultured to express the binders,thereby producing a diverse repertoire of binders. A library may bescreened for a cell of a desired phenotype, wherein the phenotyperesults from expression of a binder by a cell. Phenotype screening ispossible in which library cells are cultured to express the binders,followed by detecting whether the desired phenotype is exhibited inclones of the library. Cellular read-outs can be based on alteration incell behaviour such as altered expression of endogenous or exogenousreporter genes, differentiation status, proliferation, survival, cellsize, metabolism or altered interactions with other cells. When thedesired phenotype is detected, cells of a clone that exhibits thedesired phenotype may then be recovered. Optionally, DNA encoding thebinder is then isolated from the recovered clone, providing DNA encodinga binder which produces the desired phenotype when expressed in thecell.

A key purpose for which eukaryotic cell libraries have been used is inmethods of screening for binders that recognise a target of interest. Insuch methods a library is cultured to express the binders, and thebinders are exposed to the target to allow recognition of the target byone or more cognate binders, if present, and detecting whether thetarget is recognised by a cognate binder. In such methods, binders maybe displayed on the cell surface and those clones of the library thatdisplay binders with desired properties can be isolated. Thus cellsincorporating genes encoding binders with desired functional or bindingcharacteristics could be identified within the library. The genes can berecovered and used for production of the binder or used for furtherengineering to create derivative libraries of binders to yield binderswith improved properties.

The present invention offers advantages over previous approaches forconstruction of libraries in higher eukaryotes. Some studies have usedlentiviral infection to introduce antibody genes into mammalian reportercells [106]. This has the advantage that large libraries can begenerated but there is no control over the site of integration and copynumber is controlled by using a low multiplicity of infection (asdiscussed above). In an alternative approach antibody genes wereintroduced via homologous recombination, without the benefit ofnuclease-directed integration and using homology arms of 10 kb but theefficiency of targeting was relatively low meaning that the potentiallibrary size was limited [105]. In contrast the use of sequence-directednucleases retains the benefits of targeted integration to one or a fewloci of choice while allowing efficient construction of large libraries.Nuclease-directed integration has the advantage that transgenes aretargeted to a fixed locus or fixed loci within the cellular DNA. Thismeans that promoter activity driving transcription of binder genes inall clones will be the same and the functionality of each binder will bea reflection of its inherent potency, translational efficiency andstability rather than being due to variation related to the integrationsite. Targeting to a single or limited number of loci will also enablebetter control of expression if required e.g., using induciblepromoters.

Various features of the invention are further described below. It isnoted that headings used throughout this specification are to assistnavigation only and should not be interpreted as definitive, and thatembodiments described in different sections may be combined asappropriate.

DETAILED DESCRIPTION Eukaryotic Cells

The potential of populations of eukaryotic cells expressing a diverserepertoire of binders is exemplified and discussed in the Examplesherein in relation to expression of antibody repertoires on the surfaceof mammalian cells. The benefits of the invention are not limited tomammalian cells and include all eukaryotes.

Yeast (e.g., Saccharomyces cerevisiae) has a smaller genome thanmammalian cells and homologous recombination directed by homology arms(in the absence of nuclease-directed cleavage) is an effective way ofintroducing foreign DNA compared to higher eukaryotes. Thus, aparticular benefit of nuclease-directed integration of the presentinvention relates to integration of binder genes into higher eukaryoticcells with larger genomes where homologous recombination in the absenceof nuclease cleavage is less effective. Nuclease-directed integrationhas been used in yeast cells to solve the problem of efficientintegration of multiple genes into individual yeast cells, e.g., forengineering of metabolic pathways (US2012/0277120), but this work doesnot incorporate introduction of libraries of binders nor does it addressthe problems of library construction in higher eukaryotes.

Libraries of eukaryotic cells according to the present invention arepreferably higher eukaryotic cells, defined here as cells with a genomegreater than that of Saccharomyces cerevisiae which has a genome size of12×10⁶ base pairs (bp). The higher eukaryotic cells may for example havea genome size of greater than 2×10⁷ base pairs. This includes, forexample, mammalian, avian, insect or plant cells. Preferably the cellsare mammalian cells, e.g., mouse or human. The cells may be primarycells or may be cell lines. Chinese hamster ovary (CHO) cells arecommonly used for antibody and protein expression but any alternativestable cell line may be used in the invention. HEK293 cells are used inExamples herein. Methods are available for efficient introduction offoreign DNA into primary cells allowing these to be used (e.g., byelectroporation where efficiencies and viabilities up to 95% have beenachievedhttp://www.maxcyte.com/technology/primary-cells-stem-cells.php).

T lymphocyte lineage cells (e.g., primary T cells or a T cell line) or Blymphocyte lineage cells are among the preferred cell types. Ofparticular interest are primary T-cells or T cell derived cell lines foruse in TCR libraries including cell lines which lack TCR expression [23,24, 25]. Examples of B lymphocyte lineage cells include B cells, pre-Bcells or pro-B cells and cell lines derived from any of these.

Construction of libraries in primary B cells or B cell lines would be ofparticular value for construction of antibody libraries. Breous-Nystromet al. [15] have generated libraries in a murine pre-B cell line(1624-5). The chicken B cell derived cell line DT40 (ATCC CRL-2111) hasparticular promise for construction of libraries of binders. DT40 is asmall cell line with a relatively rapid rate of cell division.Repertoires of binders could be targeted to specific loci using ZFNs,TALE nucleases or CRISPR/Cas9 targeted to endogenous sequences or bytargeting pre-integrated heterologous sites which could includemeganuclease recognition sites. DT40 cells express antibodies and so itwill be advantageous to target antibody genes within the antibody locuseither with or without disruption of the endogenous chicken antibodyvariable domains. DT40 cells have also been used as the basis of an invitro system for generation of chicken IgMs termed the AutonomouslyDiversifying Library system (ADLib system) which takes advantage ofintrinsic diversification occurring at the chicken antibody locus. As aresult of this endogenous diversification it is possible to generatenovel specificities. The nuclease-directed approach described here couldbe used in combination with ADLib to combine diverse libraries ofbinders from heterologous sources (e.g., human antibody variable regionrepertoires or synthetically derived alternative scaffolds) with thepotential for further diversification with the chicken IgG locus.Similar benefits could apply to human B cell lines such as Nalm6 [26].Other B lineage cell lines of interest include lines such as the murinepre-B cell line 1624-5 and the pro-B cell line Ba/F3. Ba/F3 is dependenton IL-3 [27] and its use is discussed elsewhere herein. Finally a numberof human cell lines could be used including those listed in the “CancerCell Line Encyclopaedia” [28] or “COSMIC catalogue of somatic mutationsin cancer” [29].

Typically the library will be composed of a single type of cells,produced by introduction of donor DNA into a population of clonaleukaryotic cells, for example by introduction of donor DNA into cells ofa particular cell line. The main significant difference between thedifferent library clones will then be due to integration of the donorDNA.

Eukaryotic Viral Systems

The advantages of the system in creation of libraries of binders ineukaryotic cells could be applied to viral display systems based aroundeukaryotic expression systems, e.g., baculoviral display or retroviraldisplay [1, 2, 3, 4]. In this approach each cell will encode a bindercapable of being incorporated into a viral particle. In the case ofretroviral systems the encoding mRNA would be packaged and the encodedbinder would be presented on the cell surface. In the case ofbaculoviral systems, genes encoding the binder would need to beencapsulated into the baculoviral particle to maintain an associationbetween the gene and the encoded protein. This could be achieved usinghost cells carrying episomal copies of the baculoviral genome.Alternatively integrated copies could be liberated following the actionof a specific nuclease (distinct from the one used to drivesite-specific integration). In the case of multimeric binder moleculessome partners could be encoded within the cellular DNA with the genesfor one or more partners being packaged within the virus.

Site-Specific Nuclease

The invention involves use of a site-specific nuclease for targetedcleavage of cellular DNA in the construction of a library of eukaryoticcells containing DNA encoding a repertoire of binders, whereinnuclease-mediated DNA cleavage enhances site-specific integration ofbinder genes through endogenous cellular DNA repair mechanisms. Thesite-specific nuclease cleaves cellular DNA following specific bindingto a recognition sequence, thereby creating an integration site fordonor DNA. The nuclease may create a double strand break or a singlestrand break (a nick). Cells used for creation of the library maycontain endogenous sequences recognised by the site-specific nuclease orthe recognition sequence may be engineered into the cellular DNA.

The site-specific nuclease may be exogenous to the cells, i.e., notoccurring naturally in cells of the chosen type.

The site-specific nuclease can be introduced before, after orsimultaneously with introduction of the donor DNA encoding the binder.It may be convenient for the donor DNA to encode the nuclease inaddition to the binder, or on separate nucleic acid which isco-transfected or otherwise introduced at the same time as the donorDNA. Clones of a library may optionally retain nucleic acid encoding thesite-specific nuclease, or such nucleic acid may be only transientlytransfected into the cells.

Any suitable site-specific nuclease may be used with the invention. Itmay be a naturally occurring enzyme or an engineered variant. There area number of known nucleases that are especially suitable, such as thosewhich recognise, or can be engineered to recognise, sequences that occuronly rarely in cellular DNA. Nuclease cleavage at only one or two sitesis advantageous since this should ensure that only one or two moleculesof donor DNA are integrated per cell. Rarity of the sequence recognisedby the site-specific nuclease is more likely if the recognition sequenceis relatively long. The sequence specifically recognised by the nucleasemay for example be a sequence of at least 10, 15, 20, 25 or 30nucleotides.

Examples of suitable nucleases include meganucleases, zinc fingernucleases (ZFNs), TALE nucleases, and nucleic acid-guided (e.g.,RNA-guided) nucleases such as the CRISPR/Cas system. Each of theseproduces double strand breaks although engineered forms are known whichgenerate single strand breaks.

Meganucleases (also known as homing endonucleases) are nucleases whichoccur across all the kingdoms of life and recognise relatively longsequences (12-40 bp). Given the long recognition sequence they areeither absent or occur relatively infrequently in eukaryotic genomes.Meganucleases are grouped into 5 families based on sequence/structure.(LAGLIDADG, GIY-YIG, HNH, His-Cys box and PD−(D/E)×K). The best studiedfamily is the LAGLIDADG family which includes the well characterisedI-SceI meganuclease from Saccharomyces cerevisiae. I-SceI recognises andcleaves an 18 bp recognition sequence (5′ TAGGGATAACAGGGTAAT) leaving a4 bp 3′ overhang. Another commonly used example is I-Cre1 whichoriginates from the chloroplast of the unicellular green algae ofChlamydomonas reinhardtii, and recognizes a 22 bp sequence [30]. Anumber of engineered variants have been created with altered recognitionsequences [31]. Meganucleases represent the first example of the use ofsite-specific nucleases in genome engineering [49, 50]. As withrecombinase-based approaches, use of I-Sce1 and other meganucleasesrequires prior insertion of an appropriate recognition site to betargeted within the genome or engineering of meganucleases to recognizeendogenous sites [30]. By this approach targeting efficiency in HEK293cells (as judged by homology-directed “repair” of an integrateddefective GFP gene) was achieved in 10-20% of cells through the use ofI-Sce1 [32].

A preferred class of meganucleases for use in the present invention isthe LAGLIDADG endonucleases. These include I-Sce I, I-Chu I, I-Cre I,Csm I, Pl-Sce I, PI-Tli I, PI-Mtu I, I-Ceu I, I-Sce II, I-Sce III, HO,Pi-Civ I, PI-Ctr I, PI-Aae I, PI-Bsu I, PI-Dha I, PI-Dra I, PI-May I,PI-Mch I, PI-Mfu PI-Mfl I, PI-Mga I, PI-Mgo I, PI-Min I, PI-Mka I,PI-Mle I, PI-Mma I, PI-Msh I, PI-Msm I, PI-Mth I, PI-Mtu PI-Mxe I,PI-Npu I, PI-Pfu I, PI-Rma I, Pl-Spb I, PI-Ssp I, PI-Fac I, PI-Mja I,PI-Pho I, Pi-Tag I, PI-Thy I, PI-Tko I, I-Msol, and PI-Tsp I;preferably, I-Sce I, I-Cre I, I-Chu I, I-Dmo I, I-Csm I, Pl-Sce I,PI-Pfu I, PI-Tli I, PI-Mtu I, and I-Ceu I.

In recent years a number of methods have been developed which allow thedesign of novel sequence-specific nucleases by fusing sequence-specificDNA binding domains to non-specific nucleases to create designedsequence-specific nucleases directed through bespoke DNA bindingdomains. Binding specificity can be directed by engineered bindingdomains such as zinc finger domains. These are small modular domains,stabilized by Zinc ions, which are involved in molecular recognition andare used in nature to recognize DNA sequences. Arrays of zinc fingerdomains have been engineered for sequence specific binding and have beenlinked to the non-specific DNA cleavage domain of the type IIrestriction enzyme Fok1 to create zinc finger nucleases (ZFNs). ZFNs canbe used to create double stranded break at specific sites within thegenome. Fok1 is an obligate dimer and requires two ZFNs to bind in closeproximity to effect cleavage. The specificity of engineered nucleaseshas been enhanced and their toxicity reduced by creating two differentFok1 variants which are engineering to only form heterodimers with eachother [33]. Such obligate heterodimer ZFNs have been shown to achievehomology-directed integration in 5-18% of target cells without the needfor drug selection [21, 34, 35]. Incorporation of inserts up to 8 kbwith frequencies of >5% have been demonstrated in the absence ofselection.

It has recently been shown that single-stranded 5′ overhangs created bynucleases such as ZFNs help drive efficient integration of transgenes tothe sites of cleavage [45]. This has been extended to show that in vivocleavage of donor DNA (through inclusion of a specific nucleaserecognition site within the donor plasmid) enhances the efficiency onnon-homologous integration. The mechanism is not entirely clear but itis possible that reduced exposure to cellular nucleases through in vivolinearisation may have contributed to the enhancement [45]. It is alsopossible that matches in the 5′ overhangs of donor and acceptor DNA,generated by the nucleases drive ligation. Examination of sequences atthe junctions however showed the occurrence of deletions. It is possiblethat perfectly matched junctions continue to act as substrate for thesite-directed nucleases until deletion of the recognition sequenceoccurs. To overcome this potential problem, Maresca et al. [36] haveinverted the recognition sites of left and right ZFNs within the donorDNA such that ligation of donor DNA into the genomic locus will lead toduplication of two left hand ZFNs on one flank of the integration andduplication of two right hand ZFNs at the other flank. The use ofobligate heterodimer nucleases (as described for Fok1) means thatneither of these newly created flanking sequences can be cleaved by thetargeted nuclease.

The ability to engineer DNA binding domains of defined specificity hasbeen further simplified by the discovery in Xanathomonas bacteria ofTranscription activator-like effectors (TALE) molecules. These TALEmolecules consist of arrays of monomers of 33-35 amino acids with eachmonomer recognising a single base within a target sequence [37]. Thismodular 1:1 relationship has made it relatively easy to designengineered TALE molecules to bind any DNA target of interest. Bycoupling these designed TALEs to Fok1 it has been possible to createnovel sequence-specific TALE-nucleases. TALE nucleases, also known asTALENs, have now been designed to a large number of sites and exhibithigh success rate for efficient gene modification activity [38]. Inexamples herein we demonstrate the enhanced integration of donor DNAthrough the use of TALE nucleases. Other variations and enhancements ofTALE nuclease technology have been developed and could be used for thegeneration of libraries of binders through nuclease-directedintegration. These included “mega-TALENs” where a TALE nuclease bindingdomain is fused to a meganuclease [39] and “compact TALENs” where asingle TALE nuclease recognition domain is used to effect cleavage [40].

In recent years another system for directing double- or single-strandedbreaks to specific sequences in the genome has been described. Thissystem called “Clustered Regularly Interspaced Short Palindromic Repeats(CRISPR) and CRISPR Associated (Cas)” system is based on a bacterialdefence mechanism [41]. The CRISPR/Cas system targets DNA for cleavagevia a short, complementary single-stranded RNA (CRISPR RNA or crRNA)adjoined to a short palindromic repeat. In the commonly used “Type II”system, the processing of the targeting RNA is dependent on the presenceof a trans-activating crRNA (tracrRNA) that has sequence complementaryto the palindromic repeat. Hybridization of the tracrRNA to thepalindromic repeat sequence triggers processing. The processed RNAactivates the Cas9 domain and directs its activity to the complementarysequence within DNA. The system has been simplified to direct Cas9cleavage from a single RNA transcript and has been directed to manydifferent sequences within the genome [42, 43]. This approach to genomecleavage has the advantage of being directed via a short RNA sequencemaking it relatively simple to engineer cleavage specificity. Thus thereare a number of different ways to achieve site-specific cleavage ofgenomic DNA. As described above this enhances the rate of integration ofa donor plasmid through endogenous cellular DNA repair mechanisms.

Use of meganucleases, ZFNs, TALE nuclease or nucleic acid guided systemssuch as the CRISPR/Cas9 systems will enable targeting of endogenous lociwithin the genome. In the Examples herein we have demonstrated targetingto the AAVS locus but alternative loci could be targeted. For examplethe Type I collagen gene locus has been used for efficient transgeneexpression [44].

Alternatively heterologous recognition sites for targeted nucleases,including meganucleases, ZFNs and TALE nucleases could be introduced inadvance for subsequent library targeting. In Examples herein, wedescribe the use of a TALE nuclease recognising a sequence within theAAVS locus to introduce by homologous recombination, an I-Sce1meganuclease recognition sequence and heterologous TALE nucleaserecognition sites within the AAVS locus. Nuclease-directed targetingcould be used to drive insertion of target sequences by homologousrecombination or NHEJ using vector DNA or even double strandedoligonucleotides [45]. As an alternative, non-specific targeting methodscould be used to introduce targeting sites through the use oftransposon-directed integration [46] to introduce recognition sites forsite-specific nucleases. Viral-based systems, such as lentivirus,applied at low titre could also be used to introduce targeting sites.Transfection of DNA coupled with screening for single copy insertion hasalso been used to identify unique integration sites [17]. Suchnon-specific approaches would be particularly useful in the case ofcells which do not have an obvious site to target or for genomes whichhave not been sequenced or for genomes for which no existing TALEnucleases, ZFNs or Cas9/CRISPR systems are available. Once a cell linehas been established following random insertion of a nucleaserecognition site, the cell line can be used subsequently to createlibraries of binders where all clones of the library contain thetransgene at the fixed locus using nuclease-directed integration.

In the Examples presented, three different plasmids are usedencompassing pairs of TALE nucleases or ZFNs on individual plasmids witha separate plasmid for donor DNA. In the case of meganuclease thesite-specific nuclease is encoded by a single gene and this isintroduced on one plasmid with the donor DNA present on a secondplasmid. Of course, combinations could be used incorporating two or moreof these elements on the same plasmid and this could enhance theefficiency of targeting by reducing the number of number of plasmids tobe introduced. In addition it may be possible to pre-integrate thenuclease(s) which could also be inducible to allow temporal control ofnuclease activity as has been demonstrated for transposases [46].Finally the nuclease could be introduced as recombinant protein orprotein:RNA complex (for example in the case of an RNA directed nucleasesuch as CRISPR:Cas9).

Locus

A recognition sequence for the site-specific nuclease may be present ingenomic DNA, or episomal DNA which is stably inherited in the cells.Donor DNA may therefore be integrated at a genomic or episomal locus inthe cellular DNA.

In its simplest form a single gene encoding a binder (binder gene) istargeted to a single site within the eukaryotic genome. Identificationof a cell demonstrating a particular binding activity or cellularphenotype will allow direct isolation of the gene encoding the desiredproperty (e.g., by PCR from mRNA or genomic DNA). This is facilitated byusing a unique recognition sequence for the site-specific nuclease,occurring once in the cellular DNA. Cells used for creation of thelibrary may thus contain a nuclease recognition sequence at a singlefixed locus, i.e., one identical locus in all cells. Libraries producedfrom such cells will contain donor DNA integrated at the fixed locus,i.e., occurring at the same locus in cellular DNA of all clones in thelibrary.

Optionally, recognition sequences may occur multiple times in cellularDNA, so that the cells have more than one potential integration site fordonor DNA. This would be a typical situation for diploid or polyploidcells where the recognition sequence is present at correspondingpositions in a pair of chromosomes, i.e., replicate loci. Librariesproduced from such cells may contain donor DNA integrated at replicatefixed loci. For example libraries produced from diploid cells may havedonor DNA integrated at duplicate fixed loci and libraries produced fromtriploid cells may have donor DNA integrated at triplicate fixed loci.Many suitable mammalian cells are diploid, and clones of mammalian celllibraries according to the invention may have donor DNA integrated atduplicate fixed loci.

The sequence recognised by the site-specific nuclease may occur at morethan one independent locus in the cellular DNA. Donor DNA may thereforeintegrate at multiple independent loci. Libraries of diploid orpolyploid cells may comprise donor DNA integrated at multipleindependent fixed loci and/or at replicate fixed loci.

In cells containing recognition sequences at multiple loci (whetherreplicate or independent loci), each locus represents a potentialintegration site for a molecule of donor DNA. Introduction of donor DNAinto the cells may result in integration at the full number of nucleaserecognition sequences present in the cell, or the donor DNA mayintegrate at some but not all of these potential sites. For example,when producing a library from diploid cells containing recognitionsequences at first and second fixed loci (e.g., duplicate fixed loci),the resulting library may comprise clones in which donor DNA isintegrated at the first fixed locus, clones in which donor DNA isintegrated at the second fixed locus, and clones in which donor DNA isintegrated at both the first and second fixed loci.

Methods of producing libraries may therefore involve site-specificnuclease cleavage of multiple fixed loci in a cell, and integration ofdonor DNA at the multiple fixed loci. As noted above, in cases wherethere are multiple copies of the same recognition sequence (e.g., asoccurs when targeting endogenous loci in diploid or polyploid cells) itis possible that two binder genes will be integrated, particularly whenan efficient targeting mechanisms is used, with only one gene beingspecific to the target. This can be resolved during subsequent screeningonce binder genes have been isolated.

In some instances it may be desirable to introduce more than one binderper cell. For example bi-specific binders could be generated from twodifferent antibodies coming together and these may have propertiesabsent in the individual binders [47]. This could be achieved byintroducing different antibody genes into both alleles at duplicatefixed loci or by targeting different antibody populations intoindependent fixed loci using the methods described herein. Furthermore abinder may itself be composed of multiple chains (e.g., antibody VH andVL domains presented within a Fab or IgG format). In this case it may bedesirable to integrate the different sub-units into different loci.These could be integrated within the same cycle of nuclease-directedintegration, they could be integrated sequentially usingnuclease-directed integration for one or both integration steps.

Introduction of Donor DNA

Numerous methods have been described for introducing donor DNA intoeukaryotic cells, including transfection, infection or electroporation.Transfection of large numbers of cells is possible by standard methodsincluding polyethyleneimine-mediated transfection as described herein.In addition methods are available for highly efficient electroporationof 10¹⁰ cells in 5 minutes, e.g., http://www.maxcyte.com.

Combinatorial libraries could be created wherein members of multimericbinding pairs (e.g., VH and VL genes of antibody genes) or evendifferent parts of the same binder molecule are introduced on differentplasmids. Introduction of separate donor DNA molecules encoding separatebinders or binder subunits may be done simultaneously or sequentially.For example an antibody light chain could be introduced by transfectionor infection, the cells grown up and selected if necessary. Othercomponents could then be introduced in a subsequent infection ortransfection step. One or both steps could involve nuclease-directedintegration to specific genomic loci.

Integration of Donor DNA

The donor DNA is integrated into the cellular DNA, forming recombinantDNA having a contiguous DNA sequence in which the donor DNA is insertedat the integration site. In the present invention, integration ismediated by the natural DNA repair mechanisms that are endogenous to thecell. Thus, integration can be allowed to occur simply by introducingthe donor DNA into a cell, allowing the site-specific nuclease to createan integration site, and allowing the donor DNA to be integrated. Cellsmay be kept in culture for sufficient time for the DNA to be integrated.This will usually result in a mixed population of cells, including (i)recombinant cells into which the donor DNA has integrated at theintegration site created by the site-specific nuclease, and optionally(ii) cells in which donor DNA has integrated at sites other than thedesired integration site and/or optionally (iii) cells that into whichdonor DNA has not integrated. The desired recombinant cells and theresulting clones of the library may thus be provided in a mixedpopulation of other eukaryotic cells. Selection methods describedelsewhere herein may be used to enrich for cells of the library.

Endogenous DNA repair mechanisms in eukaryotic cells include homologousrecombination, non-homologous end joining (NHEJ) andmicrohomology-directed end joining. The efficiency of DNA modificationby such processes can be increased by the introduction of doublestranded breaks (DSBs) in the DNA and efficiency gains of 40,000 foldhave been reported using rare cutting endonucleases (meganucleases) suchas I-Sce1 [48, 49, 50].

Unlike the site-specific recombination involved in systems such as theFlp-In system [16], the present invention does not require exogenousrecombinases or engineered recombinase recognition sites. Therefore,optionally the present invention does not include a step ofrecombinase-mediated DNA integration in creating the library, and/oroptionally the eukaryotic cells into which the donor DNA is introducedlack a recombination site for a site-specific recombinase. Themechanisms and practicalities of directed insertion of donor DNA intocellular DNA by recombinases and nucleases are very distinct. Asdiscussed by Jasin 1996 [50]:

“ . . . the reaction catalyzed by site-specific recombinases is quitedistinct from cellular repair of DSBs. Site-specific recombinases, suchas cre, synapse two recognition sites and create single-strand breakswithin the sites, thus forming Holliday intermediates. The intermediatesare resolved to produce deletions, inversions and insertions(cointegrants), all of which restore the two recognition sites. Thereaction is absolutely precise and, hence, reversible. The breaks arenever exposed to the cellular repair machinery.”

In contrast site-specific nuclease act to create breaks or nicks withinthe cellular DNA (e.g., genomic or episomal), which are exposed to andrepaired by endogenous cellular repair mechanisms such as homologousrecombination or NHEJ. Recombinase-based approaches have an absoluterequirement for pre-integration of their recognition sites, so suchmethods require engineering of the “hot spot” integration site into thecellular DNA as a preliminary step. With nuclease-directed integrationit is possible to engineer nucleases or direct via guide RNA in the caseof CRISPR:Cas9 to recognise endogenous loci, i.e., nucleic acidsequences occurring naturally in the cellular DNA. Finally, at apractical level nuclease-directed approaches are more efficient fordirect integration of transgenes at the levels required to make largelibraries of binders.

The DNA repair mechanism by which the donor DNA is integrated in methodsof the invention can be pre-determined or biased to some extent bydesign of the donor DNA and/or choice of site-specific nuclease.

Homologous recombination is a natural mechanism used by cells to repairdouble stranded breaks using homologous sequence (e.g., from anotherallele) as a template for repair. Homologous recombination has beenutilised in cellular engineering to introduce insertions (includingtransgenes), deletions and point mutations into the genome. Homologousrecombination is promoted by providing homology arms on the donor DNA.The original approach to engineering higher eukaryotic cells typicallyused homology arms of 5-10 kb within a donor plasmid to increaseefficiency of targeted integration into the site of interest. Despitethis, homologous recombination driven purely by long homology arms, isless efficient than Flp and Cre directed recombination particularly inhigher eukaryotes with large genomes. Homologous recombination isparticularly suitable for eukaryotes such as yeast, which has a genomesize of only 12.5×10⁶ bp, where it is more effective compared withhigher eukaryotes with larger genomes e.g., mammalian cells with3000×10⁶ bp.

Homologous recombination can also be directed through [52] nicks ingenomic DNA and this could also serve as a route for nuclease-directedintegration into genomic DNA. Two distinct pathways have been shown topromote homologous recombination at nicked DNA. One is essentiallysimilar to repair at double strand breaks, utilizing Rad51/Brca2, whilethe other is inhibited by Rad51/Brca2 and preferentially usessingle-stranded DNA or nicked double stranded donor DNA [51].

Non homologous end-joining (NHEJ) is an alternative mechanism to repairdouble stranded breaks in the genome where the ends of DNA are directlyre-ligated without the need for a homologous template. Nuclease-directedcleavage of genomic DNA can also enhance transgene integration vianon-homology based mechanisms. This approach to DNA repair is lessaccurate and can lead to insertions or deletions. NHEJ nonethelessprovides a simple means of integrating in-frame exons into intron orallows integration of promoter:gene cassettes into the genome. Use ofnon-homologous methods allows the use of donor vectors which lackhomology arms thereby simplifying the construction of donor DNA.

It has been pointed out that short regions of terminal homology are usedto re-join DNA ends and it was hypothesized that 4 bp of microhomologymight be utilized for directing repairing at double strand breaks,referred to as microhomology-directed end joining [50].

Donor DNA

The donor DNA will usually be circularised DNA, and may be provided as aplasmid or vector. Linear DNA is another possibility. Donor DNAmolecules may comprise regions that do not integrate into the cellularDNA, in addition to one or more donor DNA sequences that integrate intothe cellular DNA. The DNA is typically double-stranded, althoughsingle-stranded DNA may be used in some cases. The donor DNA containsone or more transgenes encoding a binder, for example it may comprise apromoter:gene cassette.

In the simplest format double-stranded, circular plasmid DNA can be usedto drive homologous recombination. This requires regions of DNA flankingthe transgenes which are homologous to DNA sequence flanking thecleavage site in genomic DNA. Linearised double-stranded plasmid DNA orPCR product or synthetic genes could be used to drive both homologousrecombination and NHEJ repair pathways. As an alternative todouble-stranded

DNA it is possible to use single-stranded DNA to drive homologousrecombination [52]. A common approach to generating single-stranded DNAis to include a single-stranded origin of replication from a filamentousbacteriophage into the plasmid.

Single-stranded DNA viruses such as adeno-associated virus (AAV) havebeen used to drive efficient homologous recombination where theefficiency has been shown to be improved by several orders of magnitude[53, 54]. Systems such as the AAV systems could be used in conjunctionwith nuclease-directed cleavage for the construction of large librariesof binders. The benefits of both systems could be applied to targetingof libraries of binders. The packaging limit of AAV vectors is 4.7 kbbut the use of nuclease digestion of target genomic DNA will reduce thisallowing larger transgene constructs to be incorporated.

A molecule of donor DNA may encode a single binder or multiple binders.Optionally, multiple subunits of a binder may be encoded per molecule ofdonor DNA. In some embodiments, donor DNA encodes a subunit of amultimeric binder.

Promoters and Genetic Elements for Selection

Transcription of the binder from the encoding donor DNA will usually beachieved by placing the sequence encoding the binder under control of apromoter and optionally one or more enhancer elements for transcription.A promoter (and optionally other genetic control elements) may beincluded in the donor DNA molecule itself. Alternatively, the sequenceencoding the binder may lack a promoter on the donor DNA, and insteadmay be placed in operable linkage with a promoter on the cellular DNA,e.g., an endogenous promoter or a pre-integrated exogenous promoter, asa result of its insertion at the integration site created by thesite-specific nuclease.

Donor DNA may further comprise one or more further coding sequences,such as genetic elements enabling selection of cells containing orexpressing the donor DNA. As with the sequence encoding the binder,discussed above, such elements may be associated with a promoter on thedonor DNA or may be placed under control of a promoter as a result ofintegration of the donor DNA at a fixed locus. The latter arrangementprovides a convenient means of selecting specifically for those cellswhich have integrated the donor DNA at the desired site, since thesecells should express the genetic element for selection. This may be, forexample, a gene conferring resistance to a negative selection agent suchas blasticidin or puromycin. One or more selection steps may be appliedto remove unwanted cells, such as cells that lack the donor DNA or whichhave not integrated the donor DNA at the correct position. Alternativelythese cells may be permitted to remain mixed with clones of the library.

The expression of a membrane anchored binder could itself be used as aform of selectable marker. For example if a library of antibody genes,formatted as IgG or scFv-Fc fusions are introduced, then cells whichexpress the antibody can be selected using secondary reagents whichrecognise the surface expressed Fc using methods described herein. Uponinitial transfection with donor DNA encoding the transgene under thecontrol of an exogenous promoter, transient expression (and cell surfaceexpression) of the binder will occur and it will be necessary to waitfor transient expression to abate (to achieve targeted integration ofe.g., 1-2 antibody genes/cell).

As an alternative a construct encoding a membrane tethering element(e.g., the Fc domain of the present example fused to the PDGF receptortransmembrane domain) could be pre-integrated before the library ofbinders is introduced. If this membrane-tethering element lacks apromoter or is encoded within an exon which is out of frame with thepreceding exon then surface expression will be compromised. Targetedintegration of an incoming donor molecule can then correct this defect(e.g., by targeting a promoter or an “in-frame” exon into the intronwhich is upstream of the defective tethering element). If the frame“correcting exon” also encodes a binder then a fusion will be producedbetween the binder and the membrane tethering element resulting insurface expression of both. Thus correctly targeted integration willresult in-frame expression of the membrane tethering element alone or aspart of a fusion with the incoming binder. Furthermore if the incominglibrary of binders lack a membrane tethering element and these areincorrectly integrated they will not be selected. Thus expression of thebinder itself on the cell surface can be used to select the populationof cells with correctly targeted integration.

Number of Clones and Library Diversity

Yeast display libraries of 10⁷-10¹⁰ have previously been constructed anddemonstrated to yield binders in the absence of immunisation orpre-selection of the population [9, 55, 56, 57]. Many of the previouslypublished mammalian display libraries used antibody genes derived fromimmunised donors or even enriched antigen-specific B lymphocytes, giventhe limitations of library size and variability when using cells fromhigher eukaryotes. Thanks to the efficiency of gene targeting describedin the present invention large, naïve libraries can be constructed inhigher eukaryotes such as mammalian cells, which match those describedfor simpler eukaryotes such as yeast.

Following integration of donor DNA into the cellular DNA, the resultingrecombinant cells are cultured to allow their replication, generating aclone of cells from each initially-produced recombinant cell. Each cloneis thus derived from one original cell into which donor DNA wasintegrated at an integration site created by the site-specific nuclease.Methods according to the present invention are associated with a highefficiency and high fidelity of donor DNA integration, and a libraryaccording to the present invention may contain at least 100, 10³, 10⁴,10⁵, 10⁶, 10⁷, 10⁸, 10⁹ or 10¹⁰ clones.

Using nuclease-directed integration it is possible to target 10% or moreof transfected mammalian cells. It is also practical to grow andtransform >10¹⁰ cells (e.g. from 5 litres of cells growing at 2×10⁶cells/ml). Transfection of such large numbers of cells could be doneusing standard methods including polyethyleneimine-mediated transfectionas described herein. In addition methods are available for highlyefficient electroporation of 10¹⁰ cells in 5 minutes e.g.http://www.maxcyte.com. Thus using the approach of the present inventionit is possible to create libraries in excess of 10⁹ clones.

When the population of donor DNA molecules that is used to create thelibrary contains multiple copies of the same sequence, two or moreclones may be obtained that contain DNA encoding the same binder. It canalso be the case that a clone may contain donor DNA encoding more thanone different binder, for example if there is more than one recognitionsequence for the site-specific nuclease, as detailed elsewhere herein.Thus, the diversity of the library, in terms of the number of differentbinders encoded or expressed, may be different from the number of clonesobtained.

Clones in the library preferably contain donor DNA encoding one or twomembers of the repertoire of binders and/or preferably express only oneor two members of the repertoire of binders. A limited number ofdifferent binders per cell is an advantage when it comes to identifyingthe clone and/or DNA encoding a particular binder identified whenscreening the library against a given target. This is simplest whenclones encode a single member of the repertoire of binders. However itis also straightforward to identify the relevant encoding DNA for adesired binder if a clone selected from a library encodes a small numberof different binders, for example a clone may encode two members of therepertoire of binders. As discussed elsewhere herein, clones encodingone or two binders are particularly convenient to generate by selectinga recognition sequence for the site-specific nuclease that occurs onceper chromosomal copy in a diploid genome, as diploid cells containduplicate fixed loci, one on each chromosomal copy, and the donor DNAmay integrate at one or both fixed loci. Thus, clones of the library mayeach express only one or two members of the repertoire of binders.

Binders displayed on the surface of cells of the library may beidentical to (having the same amino acid sequence as) other bindersdisplayed on the same cell. The library may consist of clones of cellswhich each display a single member of the repertoire of binders, or ofclones displaying a plurality of members of the repertoire of bindersper cell. Alternatively a library may comprise some clones that displaya single member of the repertoire of binders, and some clones thatdisplay a plurality of members (e.g., two) of the repertoire of binders.

Accordingly, a library according to the present invention may compriseclones encoding more than one member of the repertoire of binders,wherein the donor DNA is integrated at duplicate fixed loci or multipleindependent fixed loci.

As noted above, it is easiest to identify the corresponding encoding DNAfor a binder if the corresponding clone expresses only one binder.Typically, a molecule of donor DNA will encode a single binder. Thebinder may be multimeric so that a molecule of donor DNA includesmultiple genes or open reading frames corresponding to the varioussubunits of the multimeric binder.

A library according to the present invention may encode at least 100,10³, 10⁴, 10⁵ or 10⁶, 10′, 10⁸, 10⁹ or 10¹⁰ different binders. Where thebinders are multimeric, diversity may be provided by one or moresubunits of the binder. Multimeric binders may combine one or morevariable subunits with one or more constant subunits, where the constantsubunits are the same (or of more limited diversity) across all clonesof the library. In generating libraries of multimeric binders,combinatorial diversity is possible where a first repertoire of bindersubunits may pair with any of a second repertoire of binder subunits.

Binders

A “binder” in accordance with the present invention is a bindingmolecule, representing a specific binding partner for another molecule.Typical examples of specific binding partners are antibody-antigen andreceptor-ligand.

The repertoire of binders encoded by a library will usually share acommon structure and have one or more regions of diversity. The librarytherefore enables selection of a member of a desired structural class ofmolecules, such as a peptide or a scFv antibody molecule. For example,the binders may be polypeptides sharing a common structure and havingone or more regions of amino acid sequence diversity.

This can be illustrated by considering a repertoire of antibodymolecules. These may be antibody molecules of a common structural class,e.g., IgG, Fab, scFv-Fc or scFv, differing in one or more regions oftheir sequence. Antibody molecules typically have sequence variabilityin their complementarity determining regions (CDRs), which are theregions primarily involved in antigen recognition. A repertoire ofbinders in the present invention may be a repertoire of antibodymolecules which differ in one or more CDRs, for example there may besequence diversity in all six CDRs, or in one or more particular CDRssuch as the heavy chain CDR3 and/or light chain CDR3.

Antibody molecules and other binders are described in more detailelsewhere herein. The potential of the present invention however extendsbeyond antibody display to include display of libraries of peptides orengineered proteins, including receptors, ligands, individual proteindomains and alternative protein scaffolds [58, 59]. Nuclease-directedsite-specific integration can be used to make libraries of other typesof binders previously engineered using other display systems. Many ofthese involve monomeric binding domains such as DARPins and lipocalins,affibodies and adhirons [58, 59, 152]. Display on eukaryotes,particularly mammalian cells, also opens up the possibility of isolatingand engineering binders or targets involving more complex, multimerictargets. For example T cell receptors (TCRs) are expressed on T cellsand have evolved to recognise peptide presented in complex with MHCmolecules on antigen presenting cells. Libraries encoding and expressinga repertoire of TCRs may be generated, and may be screened to identifybinding to MHC peptide complexes as further described elsewhere herein.

For multimeric binders, donor DNA encoding the binder may be provided asone or more DNA molecules. For example, where individual antibody VH andVL domains are to be separately expressed, these may be encoded onseparate molecules of donor DNA. The donor DNA integrates into thecellular DNA at multiple integration sites, e.g., the binder gene forthe VH at one locus and the binder gene for the VL at a second locus.Methods of introducing donor DNA encoding separate binder subunits aredescribed in more detail elsewhere herein. Alternatively, both subunitsor parts of a multimeric binder may be encoded on the same molecule ofdonor DNA which integrates at a fixed locus.

A binder may be an antibody molecule or a non-antibody protein thatcomprises an antigen-binding site. An antigen binding site may beprovided by means of arrangement of peptide loops on non-antibodyprotein scaffolds such as fibronectin or cytochrome B etc., or byrandomising or mutating amino acid residues of a loop within a proteinscaffold to confer binding to a desired target [60, 61, 62]. Proteinscaffolds for antibody mimics are disclosed in WO/0034784 in which theinventors describe proteins (antibody mimics) that include a fibronectintype III domain having at least one randomised loop. A suitable scaffoldinto which to graft one or more peptide loops, e.g., a set of antibodyVH CDR loops, may be provided by any domain member of the immunoglobulingene superfamily. The scaffold may be a human or non-human protein.

Use of antigen binding sites in non-antibody protein scaffolds has beenreviewed previously [63]. Typical are proteins having a stable backboneand one or more variable loops, in which the amino acid sequence of theloop or loops is specifically or randomly mutated to create anantigen-binding site having for binding the target antigen. Suchproteins include the IgG-binding domains of protein A from S. aureus,transferrin, tetranectin, fibronectin (e.g. 10th fibronectin type IIIdomain) and lipocalins. Other approaches include small constrainedpeptide e.g., based on“knottin” and cyclotides scaffolds [64]. Giventheir small size and complexity particularly in relation to correctformation of disulphide bond, there may be advantages to the use ofeukaryotic cells for the selection of novel binders based on thesescaffolds. Given the common functions of these peptides in nature,libraries of binders based on these scaffolds may be advantageous ingenerating small high affinity binders with particular application inblocking ion channels and proteases.

In addition to antibody sequences and/or an antigen-binding site, abinder may comprise other amino acids, e.g., forming a peptide orpolypeptide, such as a folded domain, or to impart to the moleculeanother functional characteristic in addition to ability to bindantigen. A binder may carry a detectable label, or may be conjugated toa toxin or a targeting moiety or enzyme (e.g., via a peptidyl bond orlinker). For example, a binder may comprise a catalytic site (e.g., inan enzyme domain) as well as an antigen binding site, wherein theantigen binding site binds to the antigen and thus targets the catalyticsite to the antigen. The catalytic site may inhibit biological functionof the antigen, e.g., by cleavage.

Antibody Molecules

Antibody molecules are preferred binders. Antibody molecules may bewhole antibodies or immunoglobulins (Ig), which have four polypeptidechains—two identical heavy chains and two identical light chains. Theheavy and light chains form pairs, each having a VH-VL domain pair thatcontains an antigen binding site. The heavy and light chains alsocomprise constant regions: light chain CL, and heavy chain CH1, CH2, CH3and sometimes CH4 (the fifth domain CH4 is present in human IgM andIgE). The two heavy chains are joined by disulphide bridges at aflexible hinge region. An antibody molecule may comprise a VH and/or aVL domain.

The most common native format of an antibody molecule is an IgG which isa heterotetramer consisting of two identical heavy chains and twoidentical light chains. The heavy and light chains are made up ofmodular domains with a conserved secondary structure consisting of afour-stranded antiparallel beta-sheet and a three-stranded anti-parallelbeta-sheet, stabilised by a single disulphide bond. Antibody heavychains each have an N terminal variable domain (VH) and 3 relativelyconserved “constant” immunoglobulin domains (CH1, CH2, CH3) while thelight chains have one N terminal variable domain (VL) and one constantdomains (CL). Disulphide bonds stabilise individual domains and formcovalent linkages to join the four chains in a stable complex. The VLand CL of the light chain associates with VH and CH1 of the heavy chainand these elements can be expressed alone to form a Fab fragment. TheCH2 and CH3 domains (also called the “Fc domain”) associate with anotherCH2:CH3 pair to give a tetrameric Y shaped molecule with the variabledomains from the heavy and light chains at the tips of the “Y”. The CH2and CH3 domains are responsible for the interactions with effector cellsand complement components within the immune system. Recombinantantibodies have previously been expressed in IgG format or as Fabs(consisting of a dimer of VH:CH1 and a light chain). In addition theartificial construct called a single chain Fv (scFv) could be usedconsisting of DNA encoding VH and VL fragments fused genetically withDNA encoding a flexible linker.

Binders may be human antibody molecules. Thus, where constant domainsare present these are preferably human constant domains.

Binders may be antibody fragments or smaller antibody molecule formats,such as single chain antibody molecules. For example, the antibodymolecules may be scFv molecules, consisting of a VH domain and a VLdomain joined by a linker peptide. In the scFv molecule, the VH and VLdomains form a VH-VL pair in which the complementarity determiningregions of the VH and VL come together to form an antigen binding site.

Other antibody fragments that comprise an antibody antigen-binding siteinclude, but are not limited to, (i) the Fab fragment consisting of VL,VH, CL and CH1 domains; (ii) the Fd fragment consisting of the VH andCH1 domains; (iii) the Fv fragment consisting of the VL and VH domainsof a single antibody; (iv) the dAb fragment [65, 66, 67], which consistsof a VH or a VL domain; (v) isolated CDR regions; (vi) F(ab′)2fragments, a bivalent fragment comprising two linked Fab fragments (vii)scFv, wherein a VH domain and a VL domain are linked by a peptide linkerwhich allows the two domains to associate to form an antigen bindingsite [68, 69]; (viii) bispecific single chain Fv dimers (PCT/US92/09965)and (ix) “diabodies”, multivalent or multispecific fragments constructedby gene fusion (WO94/13804; [70]). Fv, scFv or diabody molecules may bestabilised by the incorporation of disulphide bridges linking the VH andVL domains [71].

Various other antibody molecules including one or more antibodyantigen-binding sites have been engineered, including for example Fab2,Fab3, diabodies, triabodies, tetrabodies and minibodies (small immuneproteins). Antibody molecules and methods for their construction and usehave been described [72].

Other examples of binding fragments are Fab′, which differs from Fabfragments by the addition of a few residues at the carboxyl terminus ofthe heavy chain CH1 domain, including one or more cysteines from theantibody hinge region, and Fab′-SH, which is a Fab′ fragment in whichthe cysteine residue(s) of the constant domains bear a free thiol group.

A dAb (domain antibody) is a small monomeric antigen-binding fragment ofan antibody, namely the variable region of an antibody heavy or lightchain. VH dAbs occur naturally in camelids (e.g., camel, llama) and maybe produced by immunizing a camelid with a target antigen, isolatingantigen-specific B cells and directly cloning dAb genes from individualB cells. dAbs are also producible in cell culture. Their small size,good solubility and temperature stability makes them particularlyphysiologically useful and suitable for selection and affinitymaturation. Camelid VH dAbs are being developed for therapeutic useunder the name “Nanobodies™”.

Synthetic antibody molecules may be created by expression from genesgenerated by means of oligonucleotides synthesized and assembled withinsuitable expression vectors, for example as described by Knappik et al.[73] or Krebs et al. [74].

Bispecific or bifunctional antibodies form a second generation ofmonoclonal antibodies in which two different variable regions arecombined in the same molecule [75]. Their use has been demonstrated bothin the diagnostic field and in the therapy field from their capacity torecruit new effector functions or to target several molecules on thesurface of tumour cells.

Where bispecific antibodies are to be used, these may be conventionalbispecific antibodies, which can be manufactured in a variety of ways[76], e.g., prepared chemically or from hybrid hybridomas, or may be anyof the bispecific antibody fragments mentioned above. These antibodiescan be obtained by chemical methods [77, 78] or somatic methods [79, 80]but likewise and preferentially by genetic engineering techniques whichallow the heterodimerisation to be forced and thus facilitate theprocess of purification of the antibody sought [81]. Examples ofbispecific antibodies include those of the BiTE™ technology in which thebinding domains of two antibodies with different specificity can be usedand directly linked via short flexible peptides. This combines twoantibodies on a short single polypeptide chain. Diabodies and scFv canbe constructed without an Fc region, using only variable domains,potentially reducing the effects of anti-idiotypic reaction.

Bispecific antibodies can be constructed as entire IgG, as bispecificFab′2, as Fab′PEG, as diabodies or else as bispecific scFv. Further, twobispecific antibodies can be linked using routine methods known in theart to form tetravalent antibodies.

Bispecific diabodies, as opposed to bispecific whole antibodies, mayalso be particularly useful. Diabodies (and many other polypeptides,such as antibody fragments) of appropriate binding specificities can bereadily selected. If one arm of the diabody is to be kept constant, forinstance, with a specificity directed against an antigen of interest,then a library can be made where the other arm is varied and an antibodyof appropriate specificity selected. Bispecific whole antibodies may bemade by alternative engineering methods as described in Ridgeway et al.,1996 [82].

A library according to the invention may be used to select an antibodymolecule that binds one or more antigens of interest. Selection fromlibraries is described in detail below. Following selection, theantibody molecule may then be engineered into a different format and/orto contain additional features. For example, the selected antibodymolecule may be converted to a different format, such as one of theantibody formats described above. The selected antibody molecules, andantibody molecules comprising the VH and/or VL CDRs of the selectedantibody molecules, are an aspect of the present invention. Antibodymolecules and their encoding nucleic acid may be provided in isolatedform.

Antibody fragments can be obtained starting from an antibody molecule bymethods such as digestion by enzymes e.g. pepsin or papain and/or bycleavage of the disulphide bridges by chemical reduction. In anothermanner, the antibody fragments can be obtained by techniques of geneticrecombination well known to the person skilled in the art or else bypeptide synthesis by means of, for example, automatic peptidesynthesisers, or by nucleic acid synthesis and expression.

It is possible to take monoclonal and other antibodies and usetechniques of recombinant DNA technology to produce other antibodies orchimaeric molecules that bind the target antigen. Such techniques mayinvolve introducing DNA encoding the immunoglobulin variable region, orthe CDRs, of an antibody to the constant regions, or constant regionsplus framework regions, of a different immunoglobulin. See, forinstance, EP-A-184187, GB 2188638A or EP-A-239400, and a large body ofsubsequent literature.

Antibody molecules may be selected from a library and then modified, forexample the in vivo half-life of an antibody molecule can be increasedby chemical modification, for example PEGylation, or by incorporation ina liposome.

Sources of Binder Genes

The traditional route for generation of monoclonal antibodies utilisesthe immune system of laboratory animals like mice and rabbits togenerate a pool of high affinity antibodies which are then isolated bythe use of hybridoma technology. The present invention provides analternative route to identifying antibodies arising from immunisation.VH and VL genes could be amplified from the B cells of immunised animalsand cloned into an appropriate vector for introduction into eukaryoticlibraries followed by selection from these libraries. Phage display andribosome display allows very large libraries (>10⁹ clones) to beconstructed enabling isolation of human antibodies without immunisation.The present invention could also be used in conjunction with suchmethods. Following rounds of phage display selection, the selectedpopulation of binders could be introduced into eukaryotic cells bynuclease-directed integration as described herein. This would allow theinitial use of very large libraries based in other systems (e.g., phagedisplay) to enrich a population of binders while allowing theirefficient screening using eukaryotic cells as described above. Thus theinvention can combine the best features of both phage display andeukaryotic display to give a high throughput system with quantitativescreening and sorting.

Using phage display and yeast display it has previously beendemonstrated that it is also possible to generate binders withoutresorting to immunisation, provided display libraries of sufficient sizeare used. For example multiple binders were generated from a non-immuneantibody library of >10⁷ clones [83]. This in turn allows generation ofbinders to targets which are difficult by traditional immunisationroutes e.g., generation of antibodies to “self-antigens” or epitopeswhich are conserved between species. For example, human/mousecross-reactive binders can be enriched by sequential selection on humanand then mouse versions of the same target. Since it is not possible tospecifically immunise humans to most targets of interest, this facilityis particularly important in allowing the generation of human antibodieswhich are preferred for therapeutic approaches.

In examples of mammalian display to date, where library sizes andquality were limited, binders have only been generated using repertoireswhich were pre-enriched for binders, e.g., from immunisation or fromengineering of pre-existing binders. The ability to make large librariesin eukaryotic cells and particularly higher eukaryotes creates thepossibility of isolating binders direct from these libraries startingwith non-immune binders or binders which have not previously beenselected within another system. With the present invention it ispossible to generate binders from non-immune sources. This in turn opensup the possibilities for using binder genes from multiple sources.Binder genes could come from PCR of natural sources such as antibodygenes. Binder genes could also be re-cloned from existing libraries,such as antibody phage display libraries, and cloned into a suitabledonor vector for nuclease-directed integration into target cells.Binders may be completely or partially synthetic in origin. Furthermorevarious types of binders are described elsewhere herein, for examplebinder genes could encode antibodies or could encode alternativescaffolds [58, 59], peptides or engineered proteins or protein domains.

Binder Display

To provide a repertoire of binders for screening against a target ofinterest, the library may be cultured to express the binders in eithersoluble secreted form or in transmembrane form. For cell surface displayit is necessary to retain the expressed binder on the surface of thecell which encodes it. Binders may comprise or be linked to a membraneanchor, such as a transmembrane domain, for extracellular display of thebinder at the cell surface. This may involve direct fusion of the binderto a membrane localisation signal such as a GPI recognition sequence orto a transmembrane domain such as the transmembrane domain of the PDGFreceptor [84]. Retention of binders at the cell surface can also be doneindirectly by association with another cell surface retained moleculeexpressed within the same cell. This associated molecule could itself bepart of a heterodimeric binder, such as tethered antibody heavy chain inassociation with a light chain partner that is not directly tethered.

Although cell surface immobilisation facilitates selection of thebinder, in many applications it is necessary to prepare cell-free,secreted binder. It will be possible to combine membrane tethering andsoluble secretion using a recapture method of attaching the secretedbinders to cell surface receptors. One approach is to format the libraryof binders as secreted molecules which can associate with a membraneanchored molecule expressed within the same cell which can function tocapture a secreted binder. For example, in the case of antibodies orbinder molecules fused to antibody Fc domains, a membrane tethered Fccan “sample” secreted binder molecules being expressed in the same cellresulting in display of a monomeric fraction of the binder moleculesbeing expressed while the remainder is secreted in a bivalent form (U.S.Pat. No. 8,551,715). An alternative is to use a tethered IgG bindingdomain such as protein A.

Other methods for retaining secreted antibodies with the cells producingthem are reviewed in Kumar et al. (2012) [85] and include encapsulationof cells within microdrops, matrix aided capture, affinity capturesurface display (ACSD), secretion and capture technology (SECANT) and“cold capture” [85]. In examples given for ACSD and SECANT [85],biotinylation is used to facilitate immobilisation of streptavidin or acapture antibody on the cell surface. The captured molecule in turncaptures secreted antibodies. In the example of SECANT in vivobiotinylated of the secreted molecule occurs. Using the “cold capture”technique secreted antibody can be detected on producer cells usingantibodies directed to the secreted molecule. It has been proposed thatthis due to association of the secreted antibody with the glycocalyx ofthe cell [86]. Alternatively it has been suggested that the secretedproduct is trapped by staining antibodies on the cell surface beforebeing endocytosed [87]. The above methods have been used to identifyhigh expressing clones within a population but could potentially beadapted for identification of binding specificity, provided theassociation has sufficient longevity at the cell surface.

Even when the binder is directly tethered to the cell surface it ispossible to generate a soluble product. For example the gene encodingthe selected binder can be recovered and cloned into an expressionvector lacking the membrane anchored sequence. Alternatively, and asdemonstrated in the Examples, an expression construction can be used inwhich the transmembrane domain is encoded within an exon flanked byrecombination sites, e.g., ROX recognition sites for Dre recombinase[88]. In this example the exon encoding the transmembrane domain can beremoved by transfection with a gene encoding Dre recombinase to switchexpression to a secreted form. As demonstrated herein, secreted antibodywas produced by this method without the need for recombinase action.This is presumably as a result of alternative splicing [89].

Any of the above methods or other suitable approaches can be used toensure that binders expressed by clones of a library are displayed onthe surface of their expressing cells.

Screening to Identify Binders to a Target of Interest

As noted, the eukaryotic cell library may be used in a method ofscreening for a binder that recognises a target. Such a method maycomprise:

providing a library as described herein,

culturing cells of the library to express the binders,

exposing the binders to the target, allowing recognition of the targetby one or more cognate binders, if present, and

detecting whether the target is recognised by a cognate binder.

Selections could be carried using a range of target molecule classes,e.g., protein, nucleic acid, carbohydrate, lipid, small molecules. Thetarget may be provided in soluble form. The target may be labelled tofacilitate detection, e.g., it may carry a fluorescent label or it maybe biotinylated. Cells expressing a target-specific binder may beisolated using a directly or indirectly labelled target molecule, wherethe binder captures the labelled molecule. For example, cells that arebound, via the binder:target interaction, to a fluorescently labelledtarget can be detected and sorted by flow cytometry or FACS to isolatethe desired cells. Selections involving cytometry require targetmolecules which are directly fluorescently labelled or are labelled withmolecules which can be detected with secondary reagents, e.g.,biotinylated target can be added to cells and binding to the cellsurface can be detected with fluorescently labelled streptavidin such asstreptavidin-phycoerythrin. A further possibility is to immobilise thetarget molecule or secondary reagents which bind to the target on asolid surface, such as magnetic beads or agarose beads, to allowenrichment of cells which bind the target. For example cells that bind,via the binder: target interaction, to a biotinylated target can beisolated on a substrate coated with streptavidin, e.g.,streptavidin-coated beads.

In screening libraries it is preferable to over-sample, i.e., screenmore clones than the number of independent clones present within thelibrary to ensure effective representation of the library. Identifyingbinders from very large libraries provided by the present inventioncould be done by flow sorting but this would take several days,particularly if over-sampling the library.

As an alternative initial selections could be based on the use ofrecoverable antigen, e.g., biotinylated antigen recovered onstreptavidin-coated magnetic beads. Thus streptavidin-coated magneticbeads could be used to capture cells which have bound to biotinylatedantigen. Selection with magnetic beads could be used as the onlyselection method or this could be done in conjunction with flowcytometry where better resolution can be achieved, e.g., differentiatingbetween a clone with higher expression levels and one with a higheraffinity [56, 57].

The in vitro nature of display technology approaches makes it ispossible to control selection in a way that is not possible byimmunisation, e.g., selecting on a particular conformational state of atarget [90, 91]. Targets could be tagged through chemical modification(fluorescein, biotin) or by genetic fusion (e.g. protein fused to anepitope tag such as a FLAG tag or another protein domain or a wholeprotein). The tag could be nucleic acid (e.g., DNA, RNA ornon-biological nucleic acids) where the tag is part fused to targetnucleic acid or could be chemically attached to another type of moleculesuch as a protein. This could be through chemical conjugation or throughenzymatic attachment [92]. Nucleic acid could be also fused to a targetthrough a translational process such as ribosome display. The “tag” maybe another modification occurring within the cell (e.g., glycosylation,phosphorylation, ubiqitinylation, alkylation, PASylation, SUMO-lationand others described at the Post-translational Database (db-PTM) athttp://dbptm.mbc.nctu.edu.tw/statistics.php) which can be detected viasecondary reagents. This would yield binders which bind an unknowntarget protein on the basis of a particular modification.

Targets could be detected using existing binders which bind to thattarget molecule, e.g., target specific antibodies. Use of existingbinders for detection will have the added advantage of identifyingbinders within the library of binders which recognise an epitopedistinct from the binder used for detection. In this way pairs ofbinders could be identified for use in applications such as sandwichELISA. Where possible a purified target molecule would be preferred.Alternatively the target may be displayed on the surface of a populationof target cells and the binders are displayed on the surface of thelibrary cells, the method comprising exposing the binders to the targetby bringing the library cells into contact with the target cells.Recovery of the cells expressing the target (e.g., using biotinylatedcells expressing target) will allow enrichment of cells which expressbinders to them. This approach would be useful where low affinityinteractions are involved since there is the potential for a strongavidity effect.

The target molecule could also be unpurified recombinant or unpurifiednative targets provided a detection molecule is available to identifycell binding (as described above). In addition binding of targetmolecules to the cell expressing the binder could be detected indirectlythrough the association of target molecule to another molecule which isbeing detected, e.g., a cell lysate containing a tagged molecule couldbe incubated with a library of binders to identify binders not only tothe tagged molecule but also binders to its associated partner proteins.This would result in a panel of antibodies to these partners which couldbe used to detect or identify the partner (e.g., using massspectrometry). Cellular fractionation could be used to enrich targetsfrom particular sub-cellular locations. Alternatively differentialbiotinylation of surface or cytoplasmic fractions could be used inconjunction with streptavidin detection reagents for eukaryotic display[93,94]. The use of detergent solubilised target preparations is aparticularly useful approach for intact membrane proteins such as GPCRsand ion channels which are otherwise difficult to prepare. The presenceof detergents may have a detrimental effect on the eukaryotic cellsdisplaying the binders requiring recovery of binder genes withoutadditional growth of the selected cells.

Following detection of target recognition by a cognate binder, cells ofa clone containing DNA encoding the cognate binder may be recovered. DNAencoding the binder may then be isolated (e.g., identified or amplified)from the recovered clone, thereby obtaining DNA encoding a binder thatrecognises the target.

Exemplary binders and targets are detailed elsewhere herein. A classicexample is a library of antibody molecules, which may be screened forbinding to a target antigen of interest. Other examples includescreening a library of TCRs against a target MHC:peptide complex orscreening a library of MHC:peptide complexes against a target TCR.

TCR:MHC and Other Receptor Interactions

T cell receptors (TCRs) are expressed on T cells and have evolved torecognise peptide presented in complex with MHC molecules on antigenpresenting cells. TCRs are heterodimers consisting in 95% of cases ofalpha and beta heterodimers and in 5% of cases of gamma and deltaheterodimers. Both monomer units have an N terminal immunoglobulindomain which has 3 variable complementarity determining regions (CDRs)involved in driving interaction with target. The functional TCR ispresent within a complex of other sub-units and signalling is enhancedby co-stimulation with CD4 and CD8 molecules (specific for class I andclass II MHC molecules respectively). On antigen presenting cells,proteins are processed, and presented on the cell surface in complexwith MHC molecules which are themselves part of a multimeric proteincomplex. TCRs recognizing peptides originating from “self” are removedduring development and the system is poised for recognition of foreignpeptides presented on antigen presenting cells to effect an immuneresponse. The outcome of recognition of a peptide:MHC complex depends onthe identity of the T cell and the affinity of that interaction.

It would be valuable to identify the genes encoding TCRs or MHC:peptidecomplexes which drive interactions involved in pathological conditions,e.g., as occurs in autoimmune disease. In the case of autoimmunedisease, identification of interacting partners, e.g., a TCR driving apathogenic condition could pave the way to either specifically blockingsuch interactions or removing offending cytotoxic cells. It would bedesirable to engineer TCRs for altered binding e.g. higher affinity totargets of interest, e.g., in re-targeting T cells in cancer orenhancing the effect of existing T cells [95]. Alternatively thebehaviour of regulatory or suppressive T cells might be altered as atherapeutic modality, e.g., for directing or enhancing immunotherapy ofcancer by introducing specific TCRs into T cells or by using expressedTCR protein as therapeutic entities [96].

Display of libraries of TCRs on surface of yeast cells and mammaliancells has previously been demonstrated. In the case of yeast cells itwas necessary to engineer the TCR and present it in a single chainformat. Since the affinity of interaction between TCR and peptide:MHCcomplex is low, the soluble component (e.g., peptide:MHC in this case)is usually presented in a multimeric format. TCR specificity has beenengineered for peptides in complex with MHC class I [97] and MHC classII [98]. TCRs have also been expressed on the surface of a mutant mouseT cells (lacking TCR alpha and beta chains) and variant TCRs withimproved binding properties have been isolated [99]. For example Chervinet al. introduced TCRs by retroviral infection and an effective librarysize of 10⁴ clones was generated [100]. Using nuclease-directedintegration of binders as proposed here, a similar approach could betaken to engineering T cells. As well as selecting TCRs with alteredrecognition properties, display libraries could be used to screenlibraries of peptide or of MHC variants for recognition by TCRs. Forexample peptide:MHC complexes have been displayed on insect cells andused to epitope map TCRs presented in a multimeric format [101].

As noted, screening methods may involve displaying the repertoire ofbinders on the cell surface and probing with a target presented as asoluble molecule, which may be a multimeric target. An alternative,which can be especially useful with multimeric targets, is to screendirectly for cell:cell interactions, where binder and target arepresented on the surface of different cells. For example if activationof a TCR of interest led to expression of a reporter gene this could beused to identify activating peptides or activating MHC moleculespresented within a peptide:MHC library. In this particular example thereporter cell does not encode the library member but could be used toidentify the cell which does encode it. The approach could potentiallyextend to a “library versus library” approach. For example extending theexample described above, a TCR library could be screened against apeptide:MHC library. More broadly the example of screening a library ofbinders presented on one cell surface using a binding partner on anothercell could be extended to other types of cell:cell interactions e.g.,identification of binders which inhibit or activate signalling withinthe Notch or Wnt pathways. Thus the present invention could be used inalternative cell based screening system including recognition systemsbased on cell:cell interactions.

As an example, chimeric antigen receptors (“CARS”) represent a fusionbetween an antibody binding domain (usually formatted as a scFv) and asignalling domain. These have been introduced into T cells with the aimof re-directing the T cell in vivo to attack tumour cells throughantibody recognition and binding to tumour-specific antigens. A numberof different factors could affect the success of this strategy includingthe combination of antibody specificity, format, antibody affinity,linker length, fused signalling module, expression level in T cells, Tcell sub-type and interaction of the CAR with other signalling molecules[102, 103]. The ability to create large libraries of CARs in primary Tcells incorporating individual or combinations of the above variableswould allow a functional search for effective and optimal CARconstruction. This functional “search” could be carried out in vitro orin vivo. For example Alonso-Camino (2009) have fused a scFv recognizingCEA to the chain of the TCR:CD3 complex and introduced this geneticconstruct into a human Jurkat cell line [104]. Upon interaction with CEApresent on either HeLa cells or tumour cells they showed upregulation ofthe early T cell activation marker CD69. This approach could be used toidentify CAR fusion constructs with appropriate activation or inhibitoryproperties using cultured or primary cells.

Going one step further functionality of CAR constructs could be assessedin vivo. For example a library of CARs constructed in primary mouse Tcells could be introduced into tumour bearing mice to identify T cellclones stimulated to proliferate through encounter with tumour. Ifnecessary this T cell library could be pre-selected based on antigenbinding specificity using the methods described above. In either casethe incoming library of binders could be used to replace an existingbinder molecule (e.g., MHC or TCR or antibody variable domain).

Phenotype Screens

Described here are various methods for selection of binders which modifycell signalling and cellular behaviour.

A library may be screened for altered cellular phenotype as a result ofthe action of the binder on its target.

Antibodies which modify cell signalling by binding to ligands orreceptors have a proven track record in drug development and the demandfor such therapeutic antibodies continues to grow. Such antibodies andother classes of functional binders also have potential in controllingcell behaviour in vivo and in vitro. The ability to control and directcellular behaviour however relies on the availability of natural ligandswhich control specific signalling pathways. Unfortunately many naturalligands such as those controlling stem cell differentiation (e.g.,members of FGF, TGF-beta, Wnt and Notch super-families) often exhibitpromiscuous interactions and have limited availability due to their poorexpression/stability profiles. With their exquisite specificity,antibodies have great potential in controlling cellular behaviour.

The identification of functional antibodies that modify cell signallinghas historically been relatively laborious involving picking clones,expressing antibody, characterising according to sequence and bindingproperties, conversion to mammalian expression systems and addition tofunctional cell based assays. The eukaryotic display approach describedherein will reduce this effort but there is still a requirement forproduction of antibody and addition to a separate reporter cell culture.Therefore, a preferred alternative may be to directly screen librariesof binders expressed in eukaryotic cells for the effect of binding oncell signalling or cell behaviour by using the production cell itself asa reporter cell. Following introduction of antibody genes, clones withinthe resulting population of cells showing alteration in reporter geneexpression or altered phenotypes can be identified.

A number of recent publications have described the construction ofantibody libraries by cloning repertoires of antibody genes intoreporter cells [47, 105, 106]. These systems combine expression andreporting within one cell, and typically introduce a population ofantibodies selected against a pre-defined target (e.g., using phagedisplay).

A population of antibody genes may be introduced into reporter cells toproduce a library by methods described herein, and clones within thepopulation with an antibody-directed alteration in phenotype (e.g.,altered gene expression or survival) can be identified. For thisphenotypic-directed selection to work there is a requirement to retain alinkage between the antibody gene present within the expressing cell(genotype) and the consequence of antibody expression (phenotype). Thishas been achieved previously either through tethering the antibody tothe cell surface [47] as described for antibody display or through theuse of semi-solid medium to retain secreted antibodies in the vicinityof producing cells [105]. Alternatively antibodies and other binders canbe retained inside the cell [107]. Binders retained on the cell surfaceor in the surrounding medium can interact with an endogenous orexogenous receptor on the cell surface causing activation of thereceptor. This in turn can cause a change in expression of a reportergene or a change in the phenotype of the cell. As an alternative theantibody can block the receptor or ligand to reduce receptor activation.The gene encoding the binder which causes the modified cellularbehaviour can then be recovered for production or further engineering.

An alternative to this “target-directed” approach, it is possible tointroduce a “naïve” antibody population which has not been pre-selectedto a particular target [108]. The cellular reporting system is used toidentify members of the population with altered behaviour. Since thereis no prior knowledge of the target, this non-targeted approach has aparticular requirement for a large antibody repertoire, sincepre-enrichment of the antibody population to the target is not possible.This approach will benefit from using nuclease-directed transgeneintegration as described in the present invention.

The “functional selection” approach could be used on other applicationsinvolving libraries in eukaryotic cells, particularly higher eukaryotessuch as mammalian cells. The antibody could be fused to a signallingdomain such that binding to target causes activation of the receptor.Kawahara et al. have constructed chimeric receptors where anextra-cellular scFv targeting fluorescein was fused to a spacer domain(the D2 domain of the Epo receptor) and various intracellular cytokinereceptor domain including the thrombopoeitin (Tpo) receptor,erythropoietin (Epo) receptor, gp130, IL-2 receptor and the EGF receptor[109, 110, 111]. These were introduced into an IL-3 dependent proB cellline (BaF3) [27], where chimaeric receptors were shown to exhibitantigen-dependent activation of the chimaeric receptor leading to IL-3independent growth. This same approach was used in model experiments todemonstrate antigen mediated chemoattraction of BaF3 cells [110]. Theapproach was extended beyond stable culture cells to primary cellsexemplified by the survival and growth of Tpo-responsive haematopoeiticstem cells [112] or IL2 dependant primary T cells where normalstimulation by Tpo and IL-2 respectively was replaced by fluoresceindirected stimulation of scFv chimaeric receptors. Thus a system based onchimaeric antibody-receptor chimaeras can be used to drive targetdependent gene expression or phenotypic changes in primary or stablereporter cells. This capacity could be used to identify fused binderswhich drive a signalling response or binders which inhibit the response.

In a modification of the above approach separate VH and VL domains froman anti-lysozyme antibody were fused to the Epo intracellular domain[113]. Cells grew in response to addition of lysozyme indicating anantigen induced dimerisation or stabilisation of the separate VH and VLfusion partners. Thus three interacting components come together for anoptimal response in this system.

Although described here with reference to antibody molecules, the abovemethods may also be adapted and performed with libraries of otherbinders.

Protein fragment complementation represents an alternative system forstudying and for selecting protein:protein interactions in mammaliancells [114, 115]. This involves restoring function of split reporterproteins through protein:protein interactions. Reporter proteins whichhave been used include ubiquitin, DNAE intein, beta-galactosidase,dihydrofolate reductase, GFP, firefly luciferase, beta-lactamase, TEVprotease. For example a recent example of this approach is the mammalianmembrane 2 hybrid (MaMTH) approach where association of a baitprotein:split ubiquitin:transcription factor fusion with a partnerprotein:split ubiquitin restores ubiquitin recognition and liberates thetranscription factor to effect reporter gene expression [116]. Againbinders which interfere with or enhance this interaction could beidentified through perturbed signalling.

Recovery and Reformatting of Binders and Encoding DNA

Following selection of a binder or clone of interest from the library, acommon next step will be to isolate (e.g., identify or amplify) the DNAencoding the binder. Optionally, it may be desired to modify the nucleicacid encoding the binder, for example to restructure the binder and/orto insert the encoding sequence into a different vector.

Where the binder is an antibody molecule, a method may compriseisolating DNA encoding the antibody molecule from cells of a clone,amplifying DNA encoding at least one antibody variable region,preferably both the VH and VL domain, and inserting DNA into a vector toprovide a vector encoding the antibody molecule. A multimeric antibodymolecule bearing a constant domain may be converted to a single chainantibody molecule for expression in a soluble secreted form.

Antibodies may be presented in different formats but whatever format anantibody is selected in, once the antibody gene is isolated it ispossible to reconfigure it in a number of different formats. Once VH orVL domains are isolated, they can be re-cloned into expression vectorsencompassing the required partner domains (see Example 1 showing a dualpromoter IgG expression cassette).

A reformatting step may comprise reformatting of binders composed of apair of subunits (e.g., scFv molecules), to a different molecular binderformat (e.g., Ig or Fab) in which the original pairing of the subunitsis maintained. Such methods are described in more detail elsewhereherein and can be used for monoclonal, oligoclonal or polyclonal clonereformatting. The method can be used to convert “en masse” an entireoutput population from any of the commonly used display technologiesincluding phage, yeast or ribosome display.

Display of scFvs on the Surface of Mammalian Cells Fused to Fc Domains.

Although many antibody phage display libraries are formatted to displayscFvs, eukaryotic display systems will allow presentation in Fab or IgGformat. To take full advantage of the potential for IgG/Fab expression,particularly when using scFvs from other display systems will benecessary to take selected linked VH and VL domains within a bacterialexpression system and express them within a eukaryotic system fused toappropriate constant domains. Described here is a method to convert scFvpopulations to immunoglobulin (Ig) or fragment, antigen binding (Fab)format in such a way that original VH and VL chain pairings aremaintained. In the present invention, conversion is possible usingindividual clones, oligoclonal mixes or whole populations formatted asscFv while retaining the original pairing of VH and VLs chains. Themethod proceeds via the generation of an intermediate non-replicative“mini-circle” DNA which brings in a new “stuffer” DNA fragment. Thecircular DNA is linearised (e.g., by restriction digestion or PCR) whichalters the relative position of the original VH and VL fragments andplaces the “stuffer” DNA between them. Following linearization theproduct can be cloned into a vector of choice, e.g., a mammalianexpression vector. In this way all of the elements apart from the VH andVL can be replaced. Elements for bacterial expression can be replacedwith elements for mammalian expression and fusion to alternativepartners. The complete conversion process only requires a singletransformation step of E. coli bacteria to generate a population ofbacterial colonies each harbouring a plasmid encoding a unique Ig or Fabformatted recombinant antibody. Extending beyond conversion of scFv toIgG/Fab, the method can be employed to reformat any two joined DNAelements to clone into a vector such that after re-formatting each DNAelement is surrounded by different DNA control features whilstmaintaining the original pairing. A previous method has been describedwherein 2 sequential cloning steps are used [117] to replace theseelements in contrast to the present method which proceeds via anintermediate non-replicative circular intermediate.

A method of restructuring a binder, or population of binders, maycomprise converting scFv to Ig or a fragment thereof, e.g., Fab. Themethod may comprise converting nucleic acid encoding scFv to DNAencoding an immunoglobulin (Ig) or fragment thereof such as Fab format,in such a way that the original variable VH and VL chain pairings aremaintained. Preferably the conversion proceeds via circular DNAintermediate which may be a non-replicative “mini-circle” DNA. Themethod requires a single transformation of E. coli for the directgeneration of bacterial transformants harbouring plasmids encoding Ig orFab DNA.

The method may be used for monoclonal, oligoclonal or polyclonal clonereformatting. The method may be used to convert “en masse” an entireoutput population from any of the commonly used display technologiesincluding phage, yeast or ribosome display.

More generally, this aspect of the invention relates to a method thatallows the reformatting of any two joined DNA elements into a vectorwhere the DNA elements are cloned under the control of separatepromoters, or separated by alternative control elements, but maintainingthe original DNA pairing.

Examples 7 and 14 describe such methods in further detail.

Isolation, and optional restructuring, of DNA encoding binders may befollowed by introduction of that DNA into further cells to create aderivative library as described elsewhere herein, or DNA encoding one ormore particular binders of interest may be introduced into a host cellfor expression. The host cell may be of a different type compared withthe cells of the library from which it was obtained. Generally the DNAwill be provided in a vector. DNA introduced into the host cell mayintegrate into cellular DNA of the host cell. Host cells expressing thesecreted soluble antibody molecule can then be selected.

Host cells encoding one or more binders may be provided in culturemedium and cultured to express the one or more binders.

Derivative Libraries

Following production of a library by the method of the invention, one ormore library clones may be selected and used to produce a further,second generation library. When a library has been generated byintroducing DNA into eukaryotic cells as described herein, the librarymay be cultured to express the binders, and one or more clonesexpressing binders of interest may be recovered, for example byselecting binders against a target as described elsewhere herein. Theseclones may subsequently be used to generate a derivative librarycontaining DNA encoding a second repertoire of binders.

To generate the derivative library, donor DNA of the one or morerecovered clones is mutated to provide the second repertoire of binders.Mutations may be addition, substitution or deletion of one or morenucleotides. Where the binder is a polypeptide, mutation will be tochange the sequence of the encoded binder by addition, substitution ordeletion of one or more amino acids. Mutation may be focussed on one ormore regions, such as one or more CDRs of an antibody molecule,providing a repertoire of binders of a common structural class whichdiffer in one or more regions of diversity, as described elsewhereherein.

Generating the derivative library may comprise isolating donor DNA fromthe one or more recovered clones, introducing mutation into the DNA toprovide a derivative population of donor DNA molecules encoding a secondrepertoire of binders, and introducing the derivative population ofdonor DNA molecules into cells to create a derivative library of cellscontaining DNA encoding the second repertoire of binders.

Isolation of the donor DNA may involve obtaining and/or identifying theDNA from the clone. Such methods may encompass amplifying the DNAencoding a binder from a recovered clone, e.g., by PCR and introducingmutations. DNA may be sequenced and mutated DNA synthesised.

Mutation may alternatively be introduced into the donor DNA in the oneor more recovered clones by inducing mutation of the DNA within theclones. The derivative library may thus be created from one or moreclones without requiring isolation of the DNA, e.g., through endogenousmutation in avian DT40 cells.

Antibody display lends itself especially well to the creation ofderivative libraries. Once antibody genes are isolated, it is possibleto use a variety of mutagenesis approaches (e.g., error prone PCR,oligonucleotide-directed mutagenesis, chain shuffling) to create displaylibraries of related clones from which improved variants can beselected. For example, with chain-shuffling the DNA encoding thepopulation of selected VH clone, oligoclonal mix or population can besub-cloned into a vector encoding a suitable antibody format andencoding a suitably formatted repertoire of VL chains [118].Alternatively and again using the example of VHs, the VH clone, oligomixor population could be introduced into a population of eukaryotic cellswhich encode and express a population of appropriately formatted lightchain partners (e.g., a VL-CL chain for association with an IgG or Fabformatted heavy chain). The VH population could arise from any of thesources discussed above including B cells of immunised animals or scFvgenes from selected phage populations. In the latter example cloning ofselected VHs into a repertoire of light chains could combine chainshuffling and re-formatting (e.g., into IgG format) in one step.

A particular advantage of display on eukaryotic cells is the ability tocontrol the stringency of the selection/screening step. By reducingantigen concentration, cells expressing the highest affinity binders canbe distinguished from lower affinity clones within the population. Thevisualisation and quantification of the affinity maturation processusing flow cytometry is a major benefit of eukaryotic display as itgives an early indication of percentage positives in naïve library andallows a direct comparison between the affinity of the selected clonesand the parental population during sorting. Following sorting, theaffinity of individual clones can be determined by pre-incubating with arange of antigen concentrations and analysis in flow cytometry or with ahomogenous Time Resolved Fluorescence (TRF) assay or using surfaceplasmon resonance (SPR) (Biacore).

Characteristics and Form of the Library

The present invention enables construction of eukaryotic cell librarieshaving many advantageous characteristics. The invention provideslibraries having any one or more of the following features:

Diversity. A library may encode and/or express at least 100, 10³, 10⁴,10⁵ 10⁶, 10′, 10⁸ or 10⁹ different binders.

Uniform integration. A library may consist of clones containing donorDNA integrated at a fixed locus, or at a limited number of fixed loci inthe cellular DNA. Each clone in the library therefore contains donor DNAat the fixed locus or at least one of the fixed loci. Preferably clonescontain donor DNA integrated at one or two fixed loci in the cellularDNA. As explained elsewhere herein, the integration site is at arecognition sequence for a site-specific nuclease. Integration of donorDNA to produce recombinant DNA is described in detail elsewhere hereinand can generate different results depending on the number ofintegration sites. Where there is a single potential integration site incells used to generate the library, the library will be a library ofclones containing donor DNA integrated at the single fixed locus. Allclones of the library therefore contain the binder genes at the sameposition in the cellular DNA. Alternatively where there are multiplepotential integration sites, the library may be a library of clonescontaining donor DNA integrated at multiple and/or different fixed loci.Preferably, each clone of a library contains donor DNA integrated at afirst and/or a second fixed locus. For example a library may compriseclones in which donor DNA is integrated at a first fixed locus, clonesin which donor DNA is integrated at a second fixed locus, and clones inwhich donor DNA is integrated at both the first and second fixed loci.In preferred embodiments there are only one or two fixed loci in theclones in a library, although it is possible to integrate donor DNA atmultiple loci if desired for particular applications. Therefore in somelibraries each clone may contain donor DNA integrated at any one or moreof several fixed loci, e.g., three, four, five or six fixed loci.

For libraries containing binder subunits integrated at separate sites,clones of the library may contain DNA encoding a first binder subunitintegrated at a first fixed locus and DNA encoding a second bindersubunit integrated at a second fixed locus, wherein the clones expressmultimeric binders comprising the first and second subunits.

Uniform transcription. Relative levels of transcription of the bindersbetween different clones of the library is kept within controlled limitsdue to donor DNA being integrated at a controlled number of loci, and atthe same locus in the different clones (fixed locus). Relatively uniformtranscription of binder genes leads to comparable levels of expressionof binders on or from clones in a library. Binders displayed on thesurface of cells of the library may be identical to (having the sameamino acid sequence as) other binders displayed on the same cell. Thelibrary may consist of clones of cells which each display a singlemember of the repertoire of binders, or of clones displaying a pluralityof members of the repertoire of binders per cell. Alternatively alibrary may comprise some clones that display a single member of therepertoire of binders, and some clones that display a plurality ofmembers (e.g., two) of the repertoire of binders. Preferably clones of alibrary express one or two members of the repertoire of binders.

For example, a library of eukaryotic cell clones according to thepresent invention may express a repertoire of at least 10³, 10⁴, 10⁵10⁶, 10⁷, 10⁸ or 10⁹ different binders, e.g., IgG, Fab, scFv or scFv-Fcantibody fragments, each cell containing donor DNA integrated at a fixedlocus in the cellular DNA. The donor DNA encodes the binder and mayfurther comprise a genetic element for selection of cells into which thedonor DNA is integrated at the fixed locus. Cells of the library maycontain DNA encoding an exogenous site-specific nuclease.

These and other features of libraries according to the present inventionare further described elsewhere herein.

The present invention extends to the library either in pure form, as apopulation of library clones in the absence of other eukaryotic cells,or mixed with other eukaryotic cells. Other cells may be eukaryoticcells of the same type (e.g., the same cell line) or different cells.Further advantages may be obtained by combining two or more librariesaccording to the present invention, or combining a library according tothe invention with a second library or second population of cells,either to facilitate or broaden screening or for other uses as aredescribed herein or which will be apparent to the skilled person.

A library according to the invention, one or more clones obtained fromthe library, or host cells into which DNA encoding a binder from thelibrary has been introduced, may be provided in a cell culture medium.The cells may be cultured and then concentrated to form a cell pelletfor convenient transport or storage.

Libraries will usually be provided in vitro. The library may be in acontainer such as a cell culture flask containing cells of the librarysuspended in a culture medium, or a container comprising a pellet orconcentrated suspension of eukaryotic cells comprising the library. Thelibrary may constitute at least 75%, 80%, 85% or 90% of the eukaryoticcells in the container.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described in more detail, withreference to the accompanying drawings, which are as follows:

FIG. 1. Vector for expression of IgG formatted antibodies

-   -   a. pDUAL D1.3, a dual promoter expression vector for IgG        secretion (in in “pCMV/myc/ER” vector backbone)    -   b. pINT3-D1.3, a dual promoter expression vector for IgG        secretion (in “pSF-CMV-f1-Pac1” vector backbone)    -   c. pCMV/myc/ER vector backbone. ECoR1 site precedes CMV        promoter. BstB1 and BstZ171 sites flank the SV40 poly A sites.    -   d. pSF-CMV-f1-Pac1 vector backbone (Oxford Genetics)    -   e. synthetic gene with exon encoding PDGFR transmembrane region        (TM) and exon causing secretion (sec). Solid arrows represent        Rox recombination sites.

FIG. 2. Sequence of pD1 (SEQ ID NO: 1, 2, 3, 4 & 5): a dual promoterantibody expression cassette for surface expression.

Features:

pEF promoter 13-1180

BM40 leader 1193-1249

Humanised D1.3 VL 1250-1578

Human C kappa 1577-1891BGH poly A 1916-2130

CMV promoter 2146-2734

Mouse VH leader with intron 32832-3414

Humanised D1.3 VH 3419-3769

Optimised human IgG2 CH1-CH3

FIG. 3. Construction of AAVS Donor plasmid (pD2)

a. Representation of the human AAVS locus. Exon 1 and Exon 2 of the AAVSlocus (encoding protein phosphatase 1, regulatory subunit 12C, PPP1R12C)are separated by an intron of 4428 bp. Splicing is in frame “0” i.e.splicing occurs between 2 intact codons from each exon. TALENs andCRISPR/Cas9 constructs are available to cleave within this intron.Hatched blocks represent the regions to the left and right of thiscleavage site that are used within vector constructs to drive homologousrecombination into this locus (AAVS homology arm left “Left HA” and AAVShomology arm right (“right HA”).

b. Representation of the antibody encoding donor plasmid pD2. Left andright homology arms are shown at the ends of the constructrepresentation. A synthetic Blasticidin gene is preceded by a spliceacceptor which creates an “in-frame” fusion with AAVS exon 1. Also shownis the antibody expression cassette consisting of a D1.3 light chain anda D1.3 IgG2 heavy chain driven by pEF and CMV promoters respectively.

c. Sequence of donor construct pD1-huD1.3 (SEQ ID NO: 6, 7 & 8). AAVShomology arms are shown underlined and emboldened. For brevity antibodycassette (previously shown in FIG. 2) is not shown in detail.Restriction sites used in clone are shown emboldened. The sequence ofthe plasmid backbone is shown up to the ampicillin resistance gene.

FIG. 4. Expression of IgG on cell surface

a, c. Analysis was focused on viable cells using forward scatter andstaining in the FL3 channel (a, c). Cells positive for staining in theFL3 channel (representing non-viable cells which took up 7-AAD) wereexcluded. All cells were transfected with pD2-D1.3 in absence (a, b) orpresence (c.d) of the AAVS TALENs and were stained with anti-Fcantibody.

FIG. 5. Antibody binding of antigen on cell surface. Viable cells wereselected on the basis of Forward Scatter and occlusion of 7AAD (a).Cells were incubated with fluorescently labelled hen egg lysozyme (b).

FIG. 6. Effect of TALEN-directed genomic cleavage on integration ofblasticidin resistance gene. Figure shows number of colonies under theconditions described.

FIG. 7. Analysis of integration of pD2-D1.3 donor plasmid into AAVSlocus.

Following transfection cells were selected in Blasticidin and genomicDNA was prepared. Samples 1-9 benefited from addition of AAVS-directedTALE nucleases, samples 10-11 were from clones arising from celltransfected in the absence of TALE nuclease. Genomic DNA analysiscarried out by PCR as described in text.

-   -   a. Verification from 5′ end of integration site.    -   b. Verification from 3′ end of integration site.

FIG. 8. Construction of the scFv-Fc expression vector pD6

-   -   a. Antibodies formatted as scFv are cloned into the Nco1/Not1        sites to create an “in-frame” fusion with the human Fc region of        IgG2.    -   b. Sequence of pD6 from the Nco1 to Pme1 sites (SEQ ID NO: 9, 10        &11).

FIG. 9. Selection of binders from cell surface scFv-Fc library (fromselected phage populations). Flow cytometry analysis is shown of cellswith:

a-c. integrated anti-CD229 sFv-Fc population from 2 rounds of phagedisplay selection on CD229

d. f integrated anti-β-galactosidase sFv-Fc population from 1 round ofphage display selection on β-galactosidase (β-galR1 cells)

e. integrated anti-β-galactosidase sFv-Fc population from 2 rounds ofphage display selection on β-galactosidase

Sample (a) shows unstained cells and the rest were stained with humananti-Fc-phycoerythrin (in FL2) and 100 nM appropriate biotinylatedantigen/streptavidin FITC (in FL1). Cells were analysed after 13 days(a, b, d, e). Examples c and f show cells stained after 20 days and themarked region shows cells collected by flow cytometry

h. β-galR1 cells selected by flow cytometry (FIG. 6f ) were grown for 22days and re-analysed for scFv-Fc expression and antigen binding (using100 nM antigen).

g. show the unstained equivalent.

j. shows unsorted β-galR1 cells from the original population (as in d)which had been grown for 42 days after transfection (j). Unlabelledcells of each population are shown for comparison (g, i)

FIG. 10. Mammalian display and sorting of IgG formatted library.

A population of antibodies were selected on β-galactosidase using 1 or 2rounds phage display, reformatted as IgG and targeted vianuclease-directed integration into the AAVS locus of HEK293 cells.Panels a, b show cells derived from the round 1 phage population eithera, unsorted (after 38 days growth) or b, sorted by flow cytometry andgrown for 19 days. Panels c, d show cells derived from round 2 phagepopulation either c, unsorted (after 38 days growth or d, sorted andgrown for 19 days.

FIG. 11. Construction of large naïve scFv-Fc library and selection ofbinders

Cells from the naïve scFv-Fc library were stained with 500 nMbiotinylated antigen and streptavidin-FITC along withphycoerythrin-labelled anti-Fc antibody as before. Region shows cellswhich were selected by flow sorting. Samples were labelled withbiotinylated:

-   -   a. CD28    -   b. β-galactosidase    -   c. Thyroglobulin    -   d. EphB4

FIG. 12. Targeting vector to introduce intron containing “multiplelanding” sites for comparison of integration methods.

-   -   a. Intermediate GFP expression plasmid (pD3)    -   b. AAVS1_directed targeting vector (pD4). “Landing site”        incorporates elements for directing integration which are FRT,        lox 2272, I-Sce1 meganuclease and GFP TALEN

Following integration of pD4 into the AAVS locus, multiple recombinationor nuclease cleavage sites are present within the genome. The incomingpD5 plasmid (FIG. 15) has left and right homology arms equivalent to thesequence present on either side of the “landing site” to drive antibodyinsertion by homologous recombination.

FIG. 13. Sequence of pD4 (SEQ ID NO: 12, 13, 14, 15, 16, 17 & 18.).

Sequence features include:

AAVS Left homology 19-822

FRT site 832-879, Lox 2272 site 884-917, I-Sce1meganuclease site 933-950

GFP left TALEN binding 954-968, GFP right TALEN binding 984-997

T2A 1041-1103

GFP 1104-1949

PGK promoter 2178-2691

Puromycin delta thymidine kinase 2706-4307

loxP 4634-4667

AAVS right homology 4692-5528

FIG. 14. Verification of integration of “multiple landing site” intronin clone 6F.

Following transfection cells were selected in puromycin and genomic DNAwas prepared. Samples 1 represents the whole selected population. Sample2 represents clone 6F, sample 3 is a clone transfected in the absence ofTALENs and sample 4 is wild-type HEK293 cells. Primers and conditionsdescribed in text were used to verify by PCR the correct integration atthe 5′ and 3′ ends of the genomic insertion. The major (correct sized)band is seen for the selected clone (6F) as well as the selectedpopulation.

FIG. 15. Sequence of donor plasmid for integration into Flp/GFP TALENsites (pD5) (SEQ ID NO: 19, 20 & 21). Features include:

AAVS HA 13-233

FRT site 243-290

Lox2272 295-328

I-Sce1 344-361

Blasticidin resistance 417-818

Poly A 832-1070

FIG. 16. Sequence of I-Sce1 meganuclease construct (SEQ ID NO: 22 & 23)

FIG. 17. Flow cytometry analysis comparing nuclease-directed integrationusing I-Sce1 meganuclease with recombinases.

Clone 6F cells were co-transfected with pD5-D1.3 and plasmids encodingthe indicated nuclease/recombinase. Cells were selected with blasticidinand analysed 13 days after transfection using biotinylated anti-human Fcantibodies and streptavidin phycoerythrin. Percentage positive cells areindicated (also summarized in Table 5)

a. Non-transfected, b Donor only c. I-Sce1, d, eGFP TALEN, e. Cre, f.Flp recombinase (encoded by pOG44 plasmid).

FIG. 18. Nuclease-directed integration drives homologous recombinationand “non-homologous end joining” (NHEJ).

a. Representation of structure of plasmid pD5 used to target themultiple “landing site” within the intron of “clone 6F cells showingposition of primer J48. The “landing site” in this plasmid incorporatesa FRT site, a lox2272 site and an I-Sce1 meganuclease site (but no GFPTALEN site).

b. Representation of integration site within clone 6F (derived from pD4)showing position of primer J44. “Landing site” incorporates elements fordirecting integration which are FRT, lox 2272, I-Sce1 meganuclease andGFP TALEN.

c. Representation of clone 6F integration site after homologousrecombination of pD5, showing position of primers J44 and J48.

d. Representation of integration site of clone 6F after NHEJ or Flprecombination of pD5, showing position of primers J44, J46 and reverseprimer J44. The double headed arrow indicates the “extra” plasmidderived DNA incorporated by NHEJ or Flp-directed integration. Note inthis example the incoming plasmid DNA (pD5) has homology arms (whichdirect homologous recombination but are not required for NHEJ). Thesesequences are retained after integration by NHEJ, causing a duplicationof the sequence represented within the homology arms with one paircoming from the plasmid in this case and the other pair representing theendogenous genomic sequences. For simplicity the plasmid encodedhomology arms are not shown, just their equivalent sequence within thegenome.

e. Primers 44 and 48 were used as PCR primers for samples i-iv wheregenomic DNA from cells transfected with the followingnucleases/integrases were used:

i. Sce1, ii. TALEN (GFP), iii. Flp (pOG44), iv. Donor only. Molecularweight markers were “GeneRuler 1 kb ladder (New England Biolabs).Primers J44 and J48 reveal homologous recombination has occurredproducing a band of 1928 bp (indicated by arrow) in nuclease cleavedsamples i and ii.

Primers 44 and 46 were used for samples v-viii where genomic DNA fromthe following samples was used. v. Sce1, vi. TALEN (GFP), vii. Flp(pOG44), viii. Donor only.

Primers J44 and J46 reveal that cleavage of donor and genomic DNA byI-Sce1 meganuclease has resulted in NHEJ (sample v.) producing a band of1800 bp (indicated by arrow). As expected a similar sized band wasachieved by Flp mediated integration (vii). NHEJ has not occurred withGFP TALEN since there was no cleavage site in the incoming plasmid.

FIG. 19. Secretion of IgG antibodies into culture supernatant.

a. Coomassie stained gel of protein A purified IgG from culturesupernatants.

i. IgG purified from supernatant of pD2-D1.3 cells without transfectionof Dre recombinase gene.

ii. IgG purified from supernatant of pD2-D1.3 cells transfected with Drerecombinase gene.

b. Polyclonal ELISA of secreted antibodies. Sorted cells from theexperiment shown in FIG. 9H (originally from antibody population cellsselected by 1 round of phage display) were grown for 7 days post sortingand the culture supernatant collected. ELISA plates were coated witheither β-galactosidase (10 ug/ml) or BSA (10 ug/ml) overnight. Culturesupernatants were diluted down to 66% after mixing with a 33% volume of6% Marvel-PBS (this is described above as the ‘neat’ sample).Supernatant was also diluted 1/10 in PBS and mixed with 6% MPBS in thesame manner. Detection of bound scFv-Fc fusion was performed usinganti-Human IgG-Eu (Perkin Elmer Cat 1244-330).

FIG. 20. Preparation of DNA fragments for the conversion of selectedpopulations of scFv to IgG format. (a) Generation of CL-pA-CMV-Sigp DNAinsert (as depicted in FIG. 21b ). PCR amplification from plasmid pD2with primers 2595 and 2597 and gel purified. Lane m, Generuler 1 kbladder (Thermo, SM031D), lane 1 CL-pA-CMV-Sigp DNA insert. (b)Generation of scFv DNA insert was as described in Example 6. Lane m,Generuler 1 kb ladder (Thermo, SM031D), lane 1 blank, lane 2 purifiedscFv, lane 3 β-galactosidase round 1 output scFv population, lane 4β-galactosidase round 2 output scFv population, lane 5 CD229 round 2output scFv population. (c) Purification of NheI and XhoI digested“mini-circle” DNA. Ligations between Nco1/Not1 digested DNA encodingscFv (FIG. 21, insert a) and DNA encoding constant light (CO chain, polyA (pA), CMV promoter and signal peptide (FIG. 21, insert b) to form“mini-circle” DNA (FIG. 21c ) were spin column purified, digested withNheI and XhoI and purified by 1% agarose gel. Lanes are m, Generuler 1kb ladder (Thermo, SM031D), lane 1 β-galactosidase round 1 output, lane2 β-galactosidase round 2 output, lane 3 CD229 round 2 output.Linearised product at 2.6 kb, indicated by arrow, was excised andpurified.

FIG. 21. Schematic representation of the conversion process from scFv toIgG format. A DNA insert (a) encoding the antibody VH and VL domains isligated with DNA fragment (b) encoding a constant light (CO chain, apolyadenylation sequence (pA) a cytomegalovirus (CMV) promoter and asignal peptide (SigP). The joining of DNA molecules a and b to create anon-replicative DNA “mini-circle” c is facilitated by a “sticky-end”ligation. After ligation, the “mini-circle” c is linearized withrestriction enzymes NheI and XhoI. Linearized product d is then purifiedand ligated with the digested vector e. The vector e includes a pEFpromoter and SigP sequence upstream of the NheI site and encodes theantibody constant heavy (CH) domains 1 to 3 downstream of the XhoI site.The product of ligation of insert d with vector e would result inplasmid f, which can be used to transform bacteria and growth with asuitable selectable marker would allow the production and purificationof plasmid DNA by standard methods. Purified plasmid f can be introducedinto mammalian cells [134] for heterologous Ig antibody expression.Alternatively DNA encoding CH1-3 in vector e, could be replaced with DNAencoding a single CH1 domain for Fab expression. VH and VL are antibodyvariable heavy and light chain respectively. DNA encoding an elongationfactor promoter (pEF) an antibody constant light chain (CO and constantheavy domains 1 to 3 (CH1-3), a polyadenylation sequence (pA) acytomegalovirus (CMV) promoter and a signal peptide (SigP) are depicted.

FIG. 22. Additional example of preparation of DNA fragments required forthe conversion of scFv to IgG (a) scFv inserts generated as described inExample 14 were separated on a 1% agarose TBE gel. Lanes 1 and 14 is a500 bp DNA ladder starting at 500 bp. Lanes 2 to 13 are scFv PCRs. (b)The purification of the linearised “mini-circle” d (FIG. 21) wasperformed by separation on a 1% agarose TBE gel. From left to right, thefirst lane is a DNA ladder (1 kb ladder, Lifetech, 15615-024) andremaining lanes linearised “mini-circle” d. (c) As (b) except the CMVpromoter is replaced by a P2A sequence and the DNA ladder employed wasGeneruler 1 kb ladder (Thermo, SM031D).

FIG. 23. Nuclease-directed integration of binder genes using flowelectroporation systems. A 50:50 mix pD6 plasmids encoding either ananti-FGFR1 or an FGFR2 antibody was electroporated using a Flowelectroporation system. After 13 days blasticidin selection cells werelabelled with FGFR1-Fc labelled with Dyelight-633 (FGFR1-Dy633) orFGFR2-Fc labelled with Dyelight 488 (FGFR2-Dy488). Dot blots represent:

-   -   a. Single staining FGFR2-488 (sample 1b from Table 7)    -   b. single staining FGFR1-633 (sample 1b from Table 7)    -   c. Dual staining FGFR1-633/FGFR2-488 (sample 1b from Table 7)    -   d. Dual staining FGFR1-633/FGFR2-488 (sample 3 from Table 7)

FIG. 24. Recovery of antibody genes after flow sorting.

A population of antibodies was selected by one round of phage displayand a mammalian display library was created by Flow electroporationusing the Maxcyte system. Cells were sorted using either 1 nM or 10 nMantigen and mRNA isolated directly. The antibody genes were recovered byPCR, cloned into a bacterial expression vector and the proportion ofELISA positives determined (“1 nM output”, “10 nM output”). This wascompared with the original round 1 output (“R1 phage output”). Plotshows the profile of ELISA signals obtained with each population.

FIG. 25. pINT20 vector for expression of T Cell Receptors.

-   -   a. Representation of the dual promoter plasmid pINT20 showing        AAVS homology arms, puromycin selectable gene (with region        around splice acceptor site shown below). Alpha chain        (encompassing variable alpha, mouse alpha constant-CD3) is        flanked by NheI, Not1 and Acc65I restriction sites and is under        the control of pEF promoter. The beta chain (encompassing        variable beta, mouse beta constant-CD3) is flanked by Nco1, XhoI        and hind3 sites and is under the control of the CMV promoter.    -   b. Sequence at the splice acceptor and beginning of puromycin        gene (SEQ ID NO: 24 & 25)    -   c. Sequence of T cell receptor clone c12/c2 alpha chain        construct showing NheI, Not1 and Acc65I restriction sites (SEQ        ID NO: 26 & 27).    -   d. Sequence of T cell receptor clone c12/c2 beta chain construct        showing Nco1. Xho1 and Hind 3 restriction sites (SEQ ID NO: 28 &        29).    -   e. Sequence of T cell receptor clone 4JFH alpha chain construct        showing Nhe1/Not1 restriction sites (SEQ ID NO: 30 & 31).    -   f. Sequence of T cell receptor clone 4JFH beta chain construct        showing Nco1/Xho1 restriction sites (SEQ ID NO: 32 & 33).    -   g. Strategy and primer used to mutate CDR3 of c12/c2 TCR alpha        chain (SEQ ID NO: 34, 35, 36 & 37).    -   h. Strategy and primer used to mutate CDR3 of c12/c2 TCR beta        chain (SEQ ID NO: 38, 39, 40 & 41).        -   (N=A, C, G, T; S═C OR G; W=A OR T)

FIG. 26. Recognition of peptide; MHC complexes by T cell receptorsintroduced into mammalian cells by nuclease-directed integration.

TCR1 is TCR c12/c2 recognising peptide 1 (SLLMWITQV) in the form ofcomplex with phycoerythrin-labelled HLA-A2 (peptide 1). TCR2 is TCR 4JFHrecognizing peptide 2 (ELAGIGILTV) in the form of complex withphycoerythrin-labelled HLA-A2 (peptide 2). Sample a and c show cellsexpressing TCR1 exposed to peptide 1 (a) or peptide 2 (c). Sample b andd show cells expressing TCR2 exposed to peptide 1 (a) or peptide 2 (c).

Samples e and f show non-transfected HEK293 cells labelled with peptide1 (e) or peptide 2 (f).

g. Plasmid encoding TCR1 was mixed with 100 fold excess of TCR2 plasmid,introduced by nuclease-directed integration into HEK cells. 1.15% ofcells and was labelled with peptide 1. 1.15% of cells were positive.

h. Plasmid encoding TCR2 was mixed with 100 fold excess of TCR1 plasmid,introduced by nuclease-directed integration and was labelled withpeptide 2. 0.62% of cells were labelled. Positive cells collected byflow sorting and mRNA recovered for analysis of specific TCR enrichment.

Samples i-l illustrate expression of a T cell library in HEK293 cells.TCR library was introduced by Maxcyte electroporation and selected for11 days in puromycin

I. Shows cells labelled with an APC labeled anti-TCR antibody (y axis).

j. Shows cells labelled with phycoerythrin-labelled peptide 1:MHC (xaxis)

k. Shows untransfected cells labelled with both anti-TCR antibody andpeptide 1:MHC

l. Shows TCR1 library transfected cells labelled with both anti-TCRantibody and peptide 1:MHC

Samples m-n illustrate expression of TCRs in Jurkat cells. TCR1 wasdelivered by Amaxa electroporation and selected for 25 days inpuromycin. Plasmid was transfected in presence (m) or absence (n) ofTALE nuclease and was incubated with an APC labelled anti-TCR β chainantibody. Samples o-r illustrates T cell receptor activation of the sameTCR1-transfected Jurkat cells. All cells are labelled with anti-CD69antibody (y axis). Sample o was unstimulated and p was stimulated for 24hours with an anti-CD3 antibody. Samples q and r were incubated for 24hours with 2 ul and 6 ul respectively of PE labeled MHC:peptide 1. Allcells were also exposed to CD28 antibody.

FIG. 27. pINT21 CAR1 and pINT21 CAR2 vectors for introduction ofChimeric Antigen Receptor (CAR) libraries into human cells.

Representation of the single promoter plasmid pINT21 showing AAVShomology arms, puromycin selectable gene, CMV promoter driving fusion ofbinder to CD3ζ signalling domain. Nco1 and Not 1 sites are sued forcloning the binder.

-   -   a. pINT21 CAR1 fuses the binder to the juxtamembrane,        transmembrane and signalling domain of CD3.    -   b. pINT21 CAR2 fuses the binder to CD8 hinge and transmembrane        domain, 4-1BB and CD3ζ activation domains    -   c. Sequence of CD3ζ in pINT 21_CAR1 (SEQ ID NO: 42, 43 & 44)    -   d. Sequence of CD8, 4-1BB and CD3ζ in pINT 21_CAR2 (SEQ ID NO:        45 & 46)    -   e. Sequence of FMC63 H-L (anti CD19 antibody) (SEQ ID NO: 47 &        48)

FIG. 28. Expression of scFv and alternative scaffold within chimericantigen receptor construct introduced into human cells bynuclease-mediated integration.

HEK cells were transfected with anti-FGFR1 antibodies (b) or lox1adhiron (d) and labelled with labelled FGFR1 and lox1 respectively. Ascontrol the same antigens were incubated with non-transfected HEK293cells (a and c respectively).

Populations from phage display libraries selected on mesothelin andCD229 were introduced into HEK cells by nuclease-mediated integration (fand h respectively) and were selected in puromycin for 11 days. Thesecells or untransfected HEK293 cells were incubated with labelledmesothelin (e, f) or CD229 (g, h). e and h represent untransfectedHEK293 cells.

FIG. 29. Sequence of alternative binder scaffolds for mammalian displayLibraries of different binder formats can easily be introduced bynuclease-directed integration using vector described herein. By exampleAdhiron constructs were prepared with flanking Nco1 and Not 1 sites forintroduction into CAR constructs or Fc fusion constructs.

-   -   a. Sequence of lox1 binding Adhiron_lox1A (SEQ ID NO: 49 & 50)    -   b. Sequence of lox1 binding Adhiron_lox1B (SEQ ID NO: 51 & 52)

(Variable loops are shown emboldened and underlined on the proteinsequence).

-   -   c. Potential mutagenic primers for construction of library of        binders within loop 1 (adhiron mut1) (SEQ ID NO: 53, 55 & 56) or        loop 2 (adhiron mut2) (SEQ ID NO: 54, 57 & 58), Below is a        representation of the region covered by the primers (lower        strand) showing protein translation. n represents variable        number of NNS codons giving rise to different loop lengths.    -   d. Sequence of trypsin binding knottin MCoTI-II with flanking        Nco1 and Not1 sites allowing knottin expression within vectors        described herein. Sequence of first loop is underlined SEQ ID        NO: 59 & 60).    -   e. Strategy for creation of library of knottin mutants. In this        example loop 1 is replaced by 10 randomised amino acids. In this        example VNS codons are introduced (V=A, C or G) providing 24        codons encoding 17 amino acids. This sequence can be introduced        into a clone encoding MCoTI-II using standard methods (SEQ ID        NO: 61, 62 & 63).

FIG. 30. Example sequences for Nuclease mediated antibody gene insertionby ligation or microhomology-mediated end-joining (MMEJ).

a. Sequence of pD7-Sce1 (nucleotides 1-120) (SEQ ID NO: 64, 65). Thesequence is as pD6 (see FIG. 3c , 8) except the AAVS left arm betweenEcoR1 and Nsi1 has been replaced by the I-Sce1 meganuclease recognition(bold). Also the AAVS right arm between Asc1 and Mlu1 has been replacedby an insert encoded by primers 2723 and 2734 (not shown).

b. Sequence of pD7-ObLiGaRe (nucleotides 1-120) (SEQ ID NO: 66, 67). Thesequence is as pD6 (see FIG. 3c , 8) except the AAVS left arm betweenEcoR1 and Nsi1 has been replaced by the AAVS TALE right and left armrecognitions sites. Also the AAVS right arm between Asc1 and Mlu1 hasbeen replaced by an insert encoded by primers 2723 and 2734 (not shown).

FIG. 31. Nuclease directed integration of binders into the ROSA 26locus.

-   -   a. Shows sequence of left homology arm up to the beginning of        the puromycin gene showing primers and restriction sites        mentioned in example 22 (SEQ ID NO: 68).    -   b. Sequence of right homology arm for nuclease directed        integration into the ROSA 26 locus showing primers and        restriction sites mentioned in example 22 (SEQ ID NO: 69).

EXAMPLES Example 1. Construction of Vectors for Expression of IgGFormatted Antibodies

To effect genetic selections of binders (e.g. antibody, protein orpeptide) it is necessary to introduce a gene encoding this binder and todrive expression of this gene from an exogenous promoter, or bydirecting integration of the transgene downstream of a promoterpre-existing in the cellular DNA, e.g., an endogenous promoter.Antibodies represent the most commonly used class of binders and theycan be formatted for expression in different forms. In examples below,we describe expression of a single gene format where a scFv is fused toa Fc domain (scFv-Fc). We also exemplify expression of antibodiesformatted as human IgG2 molecules. To express IgG or FAb formattedantibodies in producer cells such as higher eukaryotes, it is necessaryto express the separate heavy and light chains. This can be done byintroducing separate plasmids encoding each chain or by introducing themon a single plasmid. Within a single plasmid the 2 chains can beexpressed from a multi-cistronic single mRNA. Expression of distinctproteins from a single message requires elements such as an InternalRibosome Entry (IRE) sequences which enables translation to initiate ata secondary downstream location. Alternatively, sequence elementspromoting stalling/re-initation of translation such as viral 2Asequences could be used [119].

Alternatively, multiple distinct proteins can be expressed from a singleplasmid using multiple promoters. FIGS. 1a and 1b show the organisationof 2 similar expression cassettes within different vector backbones(pDUAL and pINT3) which were developed for expression of secreted IgGformatted antibodies. These expression cassettes were created using acombination of gene synthesis and polymerase chain reactionamplification of standard elements such as promoters and poly Asequences. First separate plasmids were created within pCMV/myc/ER (FIG.1c , Life Technologies) for expression of antibody heavy chain(pBIOCAM1-NewNot) and light chain (pBIOCAM2-pEF). The elements frompBIOCAM2-pEF (including pEF promoter, light chain gene and poly A site)were cloned into pBIOCAM1-NewNot) to create pDUAL. The examples showninclude VH and VL domains from a humanised anti-lysozyme antibody calledD1.3 [120] and are referred to as pDUAL-D1.3 and pINT3-D1.3. Theelements of pDUAL D1.3 represented in FIG. 1a are present between theEcoR1 and BGH polyA site of the plasmid backbone from pCMV/myc/ER (LifeTechnologies Cat V82320 FIG. 1c ).

In a similar way separate light chain and heavy chain cassettes wereintroduced into pSF-pEF (Oxford Genetics OG43) and pSF-CMV-F1-Pac1(Oxford Genetics OG111) respectively to create pINT1 and pINT2. Thesewere combined by cloning the light chain cassette (including pEFpromoter, light chain gene and poly A site) upstream of the CMV promoterin pINT2, to create pINT3. The elements of pINT3-D1.3 represented inFIG. 1b are cloned between the first Bgl2 and the Sbf1 representedwithin the plasmid pSF-CMV-F1-Pac1 (FIG. 1d , Oxford Genetics OG111).

The immediate early promoter of cytomegalovirus (CMV promoter) is apowerful promoter and was used to drive expression of heavy chains.pDUAL D1.3 also incorporates an adenovirus 2 tripartite leader (TPL) andenhanced major late promoter (enh MLP) immediately downstream of the CMVpromoter [121]. Elongation factor-1 alpha protein is ubiquitously andabundantly expressed in most eukaryotic cells and its promoter (pEFpromoter) is commonly used for driving transgene expression [122]. InpDUAL-D1.3 and pINT3-D1.3 the pEF promoter is used to drive antibodylight chain expression. The polyadenylation sites originating in bovinegrowth hormone (BGH polyA) is present at the end of each expressioncassette.

Secretion of the separate heavy and light chains in the endoplasmicreticulum (and ultimately culture supernatant) is directed by 2different leader sequences. Light chain secretion is directed by a BM40leader sequence [123]. This is followed by NheI and Not1 cloning siteswhich allow in-frame cloning of VL genes which are in turn fused to ahuman C kappa gene. Secretion of the heavy chain is directed by a leadersplit by an intron originating from a mouse VH gene (as found inpCMV/myc/ER). The leader is followed by Nco1 and XhoI sites allowing inframe cloning of antibody VH genes followed by a codon optimised IgG2gene. The VL and VH genes of the humanised D1.3 antibody [120] werecloned into the NheI/Not1 and Nco1/XhoI sites respectively withinpDUAL-D1.3 and pINT3-D1.3.

Membrane anchored versions of these plasmids were created for mammaliandisplay. Plasmid pD1 was created by digesting pDUAL-D1.3 with Bsu36I(which cuts in CH3 domain of the IgG2 heavy chain gene) and withBstZ171, which cuts in the backbone after the SV40 poly A region of theneomycin resistance cassette (FIG. 1c ). This therefore removes most ofthe CH3 domain and the entire neomycin expression cassette. The CH3domain is replaced by a synthetic insert with compatible Bsu36I andBstZ171 ends (represented in FIG. 1e ). The synthetic insert wasdesigned to replace the stop codon at the end of the antibody CH3 domainwith a splice donor and intron which causes splicing of the CH3 terminusto an exon encoding the human PDGF receptor transmembrane domain [84]the first 5 intracellular residues, a stop codon and an additionalsplice donor. This is followed by an additional intron and spliceacceptor followed by a codon for single amino acid then a stop codon(FIG. 1e ). The 2 synthetic introns which flank the exon encoding thetransmembrane domain were designed with ROX recognition sites locatedwithin them. ROX sites are recognized by Dre recombinase causingrecombination between DNA containing these sites [88]. Inclusion of 2ROX sites flanking the transmembrane domain-encoding exon creates thepotential to remove this exon by the transfection of a gene encoding Roxrecombinase. This would be anticipated to create a secreted antibodyproduct.

FIG. 2 shows the sequence of the resulting dual promoter antibodyexpression plasmid expressing a humanised D1.3 anti-lysozyme antibody(hereafter referred to as pD1-D1.3 (SEQ ID NO: 1). Anti-lysozyme bindingspecificity is incorporated through inclusion of VH and VL sequencesfrom D1.3 [120] between Nco/Xho1 and NheI/Not1 restriction sitesrespectively. The sequence is shown from the ECoR1 site to the BstZ171.The sequences beyond the ECoR1 and BstZ171 sites are from the vectorbackbone as represented in FIG. 1 c.

Example 2. Construction of Vector (pD2) for Targeting an AntibodyCassette to the AAVS Locus

Cleavage within the genome using site-specific nucleases facilitates theinsertion of heterologous DNA through homologous recombination ornon-homologous end joining (NHEJ). Human HEK293 cells were cleaved withnucleases targeting the first intron of the protein phosphatase 1,regulatory subunit 12C (PPP1R12C) gene. This locus was identified as acommon integration site of adeno-associated virus and is referred to asthe AAVS site (FIG. 3a ). The AAVS site is considered a “safe harbour”locus for insertion and expression of heterologous genes in human cells[124].

Following site-specific cleavage within the genome it is possible topromote integration of a protein expressing cassette using homologousrecombination. To do this it is necessary to flank the expressioncassette with regions homologous to the sequences found on either sideof the genomic cleavage site. To direct integration into the AAVS locus,an 804 bp section of the AAVS locus 5′ to the intended cleavage site,was PCR amplified to create an EcoR1 and an Mfe 1 site at the 5′ and 3′end respectively. This product, representing the left homology arm fortargeting the antibody cassette, was cloned into the EcoR1 site of pD1recreating the EcoR1 site at the 5′ end. For the right homology arm an836 bp section of the AAVS locus, 3′ of the cleavage site, was PCRamplified to create Bstz171 sites at each terminus and this was clonedinto the Bstz171 of pD1. The construct is represented in FIG. 3b and thesequence of the resulting construct (pD2) is shown in FIG. 3 c.

During cloning of the AAVS left homology arm Nsi1 and Pac1 restrictionsites were also inserted at the 3′ end. These sites were subsequentlyused to clone a synthetic intron followed by a blasticidin gene with anaccompanying poly A site. The blasticidin gene lacks a promoter but ispreceded by a splice acceptor site that creates an in-frame fusion withthe upstream exon from the AAVS locus (FIG. 3a, b ). Integration intothe AAVS locus causes expression of the promoter-less blasticidin gene.The sequence of this final construct, called pD2, is shown in FIG. 3 c.

The sequence of the antibody cassette, encompassing the pEF promoter,D1.3 light chain, poly A region, CMV promoter, D1.3 heavy chain,alternative splice sites and poly A site, is shown in FIG. 2. To avoidduplication this sequence is represented in FIG. 3c as a block labelled“D1.3 ANTIBODY EXPRESSION CASSETTE”.

Example 3. AAVS TALEN-Directed Integration of IgG Construct for CellSurface Antibody Expression and Antigen Binding

HEK293F cells (Life Technologies), grown in Freestyle medium weretransfected with pD2-D1.3 DNA in the presence or absence of an AAVSdirected TALEN vector pair. An AAVS TALEN pair (“AAVS original”) waspreviously described [125] and recognises the sequence:

LEFT TALEN: (SEQ ID NO: 70) 5′ (T)CCCCTCCACCCCACAGT Spacer(SEQ ID NO: 71) 5′ GGGGCCACTAGGGAC Right TALEN: complement of(SEQ ID NO: 72) 5′ AGGATTGGTGACAGAAAA (i.e. (SEQ ID NO: 73)5′ TTTTCTGTCACCAATCCT

An alternative, more efficient AAVS targeted TALEN pair was identifiedand used in later experiments (pZT-AAVS1 L1 TALE-N and pZT-AAVS1 R1TALE, Cat No GE601A-1 System Biosciences). This pair, which recognisesthe same site (but not the first “T” residue shown in brackets above),are referred to as the “AAVS-SBI” TALEN pair.

Cells were seeded at 0.5×10⁶ cells/ml and transfected next day at 10⁶cells/ml using DNA:polyethylene imine (PolyPlus) added at a ratio of 1:2(w/w). Cells were transfected with 0.6m/ml of pD2 and wereco-transfected with either pcDNA3.0 as a control (0.6 μg/ml) or thecombined left and right “original AAVS” TALEN plasmids (0.3 μg each/ml).pD3 which expresses EGFP from the CMV promoter (see below) was includedin the experiment as a transfection control and showed 35% transfectionefficiency. Cells were selected in suspension culture using Freestylemedium (Life Technology) supplemented with 5 ug/ml blasticidin.

To determine whether antibody expression had occurred on the cellsurface, cells were stained with an anti-human Fc antibody according tothe following protocol:

1. 16 days after transfection, 0.5-1×10⁶ cells from the populationsselected with Blasticidin were centrifuged for 2 minutes (200-300×g) at4° C.2. Wash cells with 1 ml wash buffer (0.1% BSA in PBS Gibco #10010) andspin cells for 2 minutes (200-300×g) at 4° C.3. Resuspend cells in 100 μl staining buffer (1% BSA in PBS) and add5-10 ul of fluorochrome-conjugated antibodies. Antibodies werephycoerythrin-labelled anti-human IgG Fc (clone HP6017, Cat. No. 409304,Biolegend) or phycoerythrin-labelled mouse IgG2a, isotype control (Cat.No. 400214, Biolegend). Incubate for >30 min at 4° C. in the dark.4. Wash twice with 1 ml wash buffer and resuspend in 500 ul wash buffer.5. Add 5 ul of cell viability staining solution (#00-6993-50eBioscience) containing 50 ug/ml 7-amino-actinomycin D (7-AAD) toidentify dead cells.6. Cells were analysed on a (Beckton Dickinson FACS II) flow cytometer.

FIG. 4 shows that there was a significantly higher population ofantibody expressing cells when pD2-D1.3 is transfected in the presenceof the AAVS targeted TALEN with 86% positive compared with pD2-D1.3alone with 1.5% positive.

The functionality of the surface expressed anti-lysozyme antibody wasdetermined by assessing binding to labelled antigen. Hen egg lysozyme(Sigma: L6876) was labelled using Lightning-Link Rapid conjugationsystem (Dylight 488, Innova Biosciences: 322-0010) as follows:

1. Add 10 ul LL-Rapid Modifier reagent to 100 ul lysozyme (200 ugdissolved in 100 ul PBS) and mix gently.2. Add the mix to Lightning-Link® Rapid mix and resuspend gently bypipetting up and down.3. Incubate the mix for 15-30 minutes in the dark at room temperature.4. Add 10 ul LL-Rapid Quencher reagent to the reaction and mix gently.5. Store at 4° C. Final concentration of lysosyme-Dy488 is 1.6 μg/μl.6. Use 6 ul lysosyme-Dy488 (˜10 ug) per staining.7. Staining, washing and flow cytometry was as described above.

Analysis shows that 86% of cells transfected with pD2-huD1.3 boundlabelled HEL (as judged by the M1 gate) compared with 0.29% forun-transfected cells (FIG. 5).

Example 4. Site-Specific Nucleases (AAVS Directed TALENs) Enhance DonorDNA Integration

Transfected cells were also plated out and selected with blasticidin todetermine the number of cells in which expression of the promoterlessblasticidin gene was activated. 24 hours after transfection cells wereplated at 0.25×10⁶ cells/10 cm petri dish (tissue culture treated) andwere grown in 10% foetal bovine serum (10270-10⁶, Gibco) and 1% MinimalEssential medium non-essential amino acid (MEM_NEAA #11140-035 LifeTechnologies). 5 ug/ml blasticidin was added after another 24 hours andmedium was changed every 2 days. After 9 days cells which did notreceive pD2 plasmid were all dead. After 12 days plates were stainedwith 2% methylene blue (in 50% methanol). Colony density was too highfor accurate quantitation but showed an increased number of blasticidinresistant colonies in the presence of the AAVS TALENs suggestingtargeted integration into the AAVS locus. A reduced amount of DNA wasintroduced for more accurate quantitation.

Transfections were carried out as described earlier using either 50, 200or 400 ng pD2-D1.3/10⁶ cells in presence or absence of the AAVS TALENs(0.3 ug/ml of each TALEN where present, Table 1A). The total DNA inputwas adjusted to 1.2 ug DNA per 10⁶ cells with control plasmid pcDNA3.0.After 24 hrs of transfection, 0.25×10⁶ cells were plated in a 10 cm dishand 7.5 ug/ml blasticidin was added after 24 hrs of plating. 10 daysafter blasticidin selection the colonies are stained with 2% methyleneblue (in 50% methanol). Results are shown in FIG. 6 and summarised inTable 1A. This shows that co-transfection of DNA encoding AAVS-directedTALENs increases the number of blasticidin resistant colonies achievedby approximately 10 fold.

A comparison was carried out between “AAVS original” and the “AAVS SBI”TALEN pairs targeting the AAVS locus. Table 1B shows an increased numberof blasticidin resistant colonies using the “AAVS SBI” TALEN pair.

TABLE 1 Quantitation of blasticidin-resistant colonies from transfectionof pD2-D1.3 A. pD2-D1.3 donor With AAVS Without AAVS Enzyme plasmid(ng/10⁶ cells) TALEN TALEN AAVS original 50 319 32 AAVS original 200 52641 AAVS original 400 686 75 B. Without AAVS pD2-D1.3 donor With AAVSTALEN (Control Enzyme plasmid (ng/10⁶ cells) TALEN pcDNA3.0) AAVSoriginal 300 1420 111 AAVS original 1000 1080 127 AAVS original 3000 56070 AAVS-SBI 300 2800 111 AAVS-SBI 1000 1630 127 AAVS-SBI 3000 870 70

Here we have compared the effect of TALEN nuclease addition using eithercell surface antibody expression (Example 3) or activation of apromoter-less blasticidin gene (Example 4). The benefit ofnuclease-directed integration is more obvious when measuring antibodyexpression compared to effect on blasticidin-resistant colonies. Onelikely explanation is that the levels of expression required to effectsurvival in the presence of blasticidin may be significantly less thanthe expression levels required to detect IgG2 expression on the surface.Thus misincorporation/splicing of the promoter-less blasticidin genecould lead to a low level expression of the blasticidin resistance genecausing a higher background of blasticidin resistant colonies in theabsence of significant antibody expression.

Example 5. Determination of Accuracy of Integration Using AAVS TALEN

To investigate the accuracy of integration, colonies were picked fromthe experiment in Example 4/Table 1A (from duplicate, unstained plates),expanded and genomic DNA from these cells was used as template in PCR.For preparation of genomic DNA, cells were harvested and werere-suspended in 700 μL of lysis buffer (10 mM Tris.Cl, pH=8.0, 50 mMEDTA, 200 mM NaCl, 0.5% SDS, supplemented with 0.5 mg/mL of Proteinase K(added just before lysis). The cell re-suspension in lysis buffer wasthen transferred to a microfuge tube and kept at 60° C. for about 18hours. Next day, 700 μL of isopropanol was added to the lysate in orderto precipitate genomic DNA. The microfuge tube was spun at 13,000 rpmfor 20 minutes. The genomic DNA pellet was then washed with 70% ethanol,and spun at 13,000 rpm for another 10 minutes. After spinning, thesupernatant was carefully separated taking care not to touch the genomicDNA pellet. The genomic DNA pellet was then re-suspended in 100 μLbuffer containing 10 mM Tris (pH 8.0), and 1 mM EDTA and kept at 60° C.for 30 minutes keeping the lid open in order to get rid of traces ofethanol. To this 100 μL solution, RNAse A was added (final concentrationof 20 μg/mL), and incubated at 60° C. for about one hour. Genomic DNAconcentration was measured using nanodrop spectrophotometer (Nanodrop).

To identify correct integration, PCR primers were designed whichhybridise in the AAVS genomic locus beyond the left and right homologyarms. These were paired up with insert specific primers. At the 5′ endthe primers were:

AAVS-Left-arm-junction-PCR-Forw (9625) (SEQ ID NO: 74)5′ CCGGAACTCTGCCCTCTAAC BSD_Junction PCR-rev (9626): (SEQ ID NO: 75)5′ TAGCCACAGAATAGTCTTCGGAG

These give a product of 1.1 kb where correct integration occurs. 8/9clones arising from AAVS directed integration gave a band of correctsize (FIG. 7a, b ). 2 blasticidin resistant clones derived withoutTALENs did not give a product (FIG. 11a ) indicative of randomintegration. At the 3′ end primers were:

Donor_plasmid_seq_PDGFRTM-2 Forw (SEQ ID NO: 76)5′ ACACGCAGGAGGCCATCGTGG AAVS1_right arm_junction_PCR_rev(SEQ ID NO: 77) 5′ TCCTGGGATACCCCGAAGAG

These give a product of 1.5 kb with correct integration. 7/9 clonesarising from AAVS directed integration gave a band of correct size. 2blasticidin resistant clones derived without TALENs did not give aproduct (FIG. 11b ). Thus the majority of blasticidin resistant cellsarise from correct integration into the AAVS locus whereas blasticidinresistant colonies arising in the absence of TALENs are not correctlyintegrated.

Example 6. Construction of an scFv Display Library from a SelectedPopulation from Phage Display and Selection Via Mammalian Display

scFv formatted soluble antibodies have previously been expressed fromthe vector pBIOCAM5-3F where expression is driven by the CMV promoterand the vector provides a C-terminal fusion partner, consisting of humanFc, His6 and 3×FLAG, to the antibody gene [105, 126]. This was modifiedto create the vector pBIOCAM5newNot where the Not1 site was embeddedwithin the Fc region of the antibody (as shown in FIG. 8). This was usedas a starting point to create the vector pD6 (FIG. 8) for expression ofscFv-Fc fusions tethered to the cell surface. Primers (2598 and 2619)were designed to allow amplification of the CMV promoter-scFv-Fcexpression cassette from pBIOCAM5newNotPrimer 2598 hybridises upstreamof the CMV promoter and places a Pac1 site (bold text) at the end.

2598: (SEQ ID NO: 78) TTTTTTTTAATTAA GATTATTGACTAGTTATTAATAGTAATCAATTACGGGGTC

Primer 2619 hybridises near the end of the Fc domain and introduces aslice donor site and Pme1 site (bold text) at the beginning of theintron.

2619: (SEQ ID NO: 79) TTTTTTGTTTAAACTTACCTTGGATCCCTTGCCGGGGCTCAGGCTCAGGGAC

The resulting PCR product is compatible with the Pac1 and Pme1 sites ofpD2 (FIG. 3).

Digestion of pD2 with Pac1 and Pme1 removes: pEF promoter-leader-lightchain-CMV promoter-leader-heavy chainCloning of the Pac1/Pme1 cut PCR product insets: CMVpromoter-leader-Nco1/Not1 sites-human Fc.

Cloning in this way positions the scFv-Fc cassette appropriately forsplicing to the downstream trans-membrane domain previously describedfor IgG presentation on the cell surface in pD2. The final vector pD6 isshown in FIG. 8, the sequence of D6 from Nco1 to Pme1 sites is shown.

Phage display selections were carried out using the McCafferty phagedisplay library [7] using beta-galactosidase (Rockland, Cat B000-17) andCD229 (R and D Systems, Cat 898-CD-050) as antigens. Methods forselection and sub-cloning were essentially as described previously [6,7, 118, 127]. scFv genes from populations arising from one or two roundsof selection on beta-galactosidase and two rounds of selection on CD229were recovered by PCR. Primers M13Leadseq hybridises within thebacterial leader sequence preceding the scFv gene and Notmycseqhybridises in the myc tag following the scFv gene in the phage displayvector [127].

M13Leadseq  (SEQ ID NO: 80) AAA TTA TTA TTC GCA ATT CCT TTG GTT GTT CCTNotmycseq  (SEQ ID NO: 81) GGC CCC ATT CAG ATC CTC TTC TGA GAT GAG

PCR product was digested with Nco1 and Not1, the digested insert was gelpurified. The digested product was ligated into the Nco1 and Not 1 sitesof the bacterial expression plasmid pSANG10-3F and antibody expressedand screened as described [127]. After 2 rounds of selection onbeta-galactosidase and CD229, 40/190 (21%) and 35/190 (18%) clones werefound to be positive by ELISA.

550 ng of Nco/Not cut insert was also ligated into the Nco1 and Not 1sites of pD6 (2.4 μg) to create a construct expressing a fusion betweenthe scFv and the Fc region of human IgG2. Ligated DNA was transformedinto electro-competent NEB5alpha cells (New England Biolabs, Cat C2989)which generated a library size of 2-3×10⁷ clones for each population.DNA was prepared and was co-transfected into 100 mls HEK293 cells grownin Freestyle medium as described above using 0.3 μg donor DNA(pD6-library) per 10⁶ cells. Cells were co-transfected with 0.5 μg eachof “AAVS-SBI” TALENs (pZT-AAVS1 L1 TALE-N and pZT-AAVS1 R1 TALE, Cat NoGE601A-1 System Biosciences).

24 hours after transfection the volume of the bulk culture was doubledand 24 hours later blasticidin (10 μg/ml) was added. Medium wasrefreshed every 3-4 days and after 6 days blasticidin concentration wasincreased to 20 μg/ml.

In order to determine the library size, 20,000 cells were plated in a 10cm petri dish (tissue culture treated) 24 hours after transfection andwere grown in 10% foetal bovine serum (10270-106, Gibco) and 1% MinimalEssential medium non-essential amino acid (MEM_NEAA #11140-035 LifeTechnologies). 10 μg/ml blasticidin was added after another 24 hours andmedium was changed every 2 days. After 8 days plates were stained with2% methylene blue (in 50% methanol). Results are shown in Table 2. Thisshows that libraries of around 3×10⁶ clones (representing 3% oftransfected cells) were obtained for the 3 populations.

TABLE 2 Determination of scFv-Fc library size. No colonies/ No colonies/Library Sample 20,000 cells 10⁶ cells size β-galactosidase Rd1 54627,300 2.7 × 10⁶ β-galactosidase Rd2 654 32,700 3.2 × 10⁶ CD229 Rd2 55627,800 2.8 × 10⁶

The protocol for labelling and flow sorting 10-20×10⁶ cells is shownbelow. Initial analysis was carried out 13 days post-transfection usingonly 10⁶ cells/sample and with reduced incubation volumes (reagentvolumes that are 1/10th of those shown).

FIG. 9 shows that at 13 days post-transfection at least 43-46% of cellsexpress scFv-Fc fusion on the cell surface and this can be detectedusing either FITC or phycoerythrin-labelled anti-Fc antibodies. Bindingof biotinylated beta-galactosidase is also detected within thispopulation using either FITC or phycoerythrin-labelled streptavidin.Using streptavidin-FITC 11.8% and 39% of the cell were positive for bothantibody expression and antigen binding using libraries derived fromoutput populations arising from 1 or 2 rounds of phage display selectionrespectively. For CD229 derived from 2 rounds of phage display, 66% ofcells were positive for scFv-Fc and 24% of these were positive for CD229binding (15% of the total population).

At 20 days after transfection cells were labelled according to theprotocol below (using biotinylated antigen/phycoerythrin-labelledstreptavidin and FITC-labelled anti human Fc).

1. Harvest, wash and adjust cells in 15-20×10⁶ cells per sample. Spindown cells at 250 g for 4′, RT, wash cells with 1 ml PBS+0.1% BSA (4°C.), spin down cells at 250 g for 4′, RT, resuspend in 1 ml PBS+1% BSA2. Add biotinylated antigen to a final conc. 100 nM and incubate 30′ at4° C.3. Wash the cells 2 times 1 ml of 0.1% BSA by centrifugation at 1500 rpmfor 5 minutes4. Add either:

10 μl of FITC-labelled streptavidin (1 μg/ml, Sigma Cat S3762) and 20 μlof phycoerythrin-labelled anti human Fc (200 μg/ml, BioLegend Cat.409304), or:

20 μl phycoerythrin-labelled streptavidin (200 μg/ml, Biolegend Cat405203) and 20 μl of FITC-labelled anti human Fc (200 μg/ml, BiolegendCat 409310) PBS+1% BSA, for 15 at 4° C. in the dark

5. Wash the cells 2 times 1 ml of 0.1% BSA by centrifugation at 1500 rpmfor 5 minutes6. resuspend them in 500 μl ice cold PBS+1% BSA7. Add 20 ul of 7AAD/vial for viability staining

For sorting cells were gated on the basis of cell size, granularity,pulse width and viability (via 7-AAD staining, forward scatter and sidescatter. Results are shown in FIGS. 9c and f . In total 10 million cellswere sorted and 3.1% and 7% of doubly positive cells were collected forlibraries derived from output populations arising from 2 rounds of CD229(CD229 R2) selection and 1 round of β-galactosidase selection (β-galR1)respectively.

Selected cells from the β-galR1-derived cells were grown for a further20 days and re-analysed (FIG. 9h ). This shows that the majority ofcells now express scFv-Fc and bind β-galactosidase. This figure alsoshows that the proportion of double positive cells within the unselectedpopulation has not diminished 42 days after transfection (FIG. 9k ).

Genomic DNA was prepared from 150,000-10⁶ sorted cells. Genomic DNA wasprepared using method described earlier or using a GenElute mammaliangenomic DNA miniprep kit (Sigma G1N10).

scFv genes were PCR amplified from genomic DNA using the followingprimers:

2623  (SEQ ID NO: 82) TAAAGTAGGCGGTCTTGAGACG 2624  (SEQ ID NO: 83)GAAGGTGCTGTTGAACTGTTCC

PCR reactions were carried out using Phusion polymerase (NEB Cat M0532S)in manufacturer's buffer containing 0.3 uM of each primer and 3% DMSO.100-1000 ng of genomic DNA was used as template in a 50 ul reaction. 30cycles were carried out at 98° C. for 10 secs, 55° C. for 25 secs, 72°C. for 45 secs. This gave a product of 1.4 kB which was digested withNco1 and Not1. A band of approximately 750-800 bp was generated and gelpurified before cloning into pSANG10. Ligated DNA was transformed intoBL21 cells (Edge Bio Ultra BL21 (DE3) competent cells, Cat. 45363). Inthis way scFv fragments derived from the sorted population can beexpressed in bacteria as described previously [7, 127].

As an alternative to isolating the antibody gene and expressing in analternative vector/host combination, it is possible to derive secretedantibody directly from the selected cells either following single cellcloning or using a sorted population to generate a polyclonal antibodymix. To exemplify this culture supernatant was taken from sorted cells(from β-galR1 cells) after 7 days in culture. This was shown to bepositive in ELISA using plates coated with β-galactosidase (see Example13 and FIG. 19b ).

Example 7. Construction and Selection from an IgG Display Library from aSelected Population from Phage Display

DNA fragments encoding scFv, representing the round 1 and 2 antibodyphage display outputs of selections against β-galactosidase and CD229,were generated as described in Example 6. The scFv populations wereconverted to IgG format according to Example 14 and as detailed in themethod below.

A DNA insert encoding the human kappa light chain constant domain (CL),polyadenylation sequence (pA), CMV promoter and signal peptide frommurine VH chain (represented between the Not1 and Nco1 sites of pD2shown in FIG. 21b ) was PCR amplified from plasmid pD2 with primers 2595(GAGGGCTCTGGCAGCTAGC) (SEQ ID NO: 84) and 2597 (TCGAGACTGTGACGAGGCTG)(SEQ ID NO: 85). PCR reactions were carried out using KOD hot startpolymerase (Novagen Cat 71086-4) in manufacturer's buffer containing0.25 μM of each primer. 10 ng of pD2 plasmid DNA was used as template ina 50 ul reaction. 25 cycles were carried out at 98° C. for 10 secs, 55°C. for 25 secs, 72° C. for 40 secs. This gave a product of 1.8 kB whichwas digested with Nco1 and Not1 and gel purified (FIG. 20a representedas CL-pA-CMV-SigP insert in FIG. 21b ).

DNA fragments encoding scFv, representing the round 1 and 2 antibodyphage display outputs of selections against β-galactosidase and CD229,were generated as described in Example 6. FIG. 20b shows scFvpopulations selected against β-galactosidase and CD229 separated by 1%agarose gel electrophoresis.

Ligations between scFv insert and CL-pA-CMV-SigP insert were performedby incubating Nco1/Not1 digested scFv insert (1 μg) with Nco1/Not1digested CL-pA-CMV-SigP insert (1 μg) with T4 DNA ligase (1.5 μl, Roche,10-481-220-001) in manufacturer's buffer in a total volume of 40 μl toform the “mini-circle” depicted in FIG. 21c . Ligations were incubatedat 16° C. for 16 hours, purified by spin column, digested with NheI andXhoI and the 2.6 kb product (depicted in FIG. 21d ) purified byelectrophoretic separation on 1% agarose gel (FIG. 20c ).

The DNA insert depicted in FIG. 21d encoding VL-CL-pA-CMV-SigP-VH (0.5μg) was ligated with NheI/XhoI digested, gel purified vector pD2 (0.7μg) (FIG. 21e ) with T4 DNA ligase (1.5 μl, Roche, 10-481-220-001) inmanufacturer's buffer in a total volume of 40 μl to produce thetargeting vector depicted in FIG. 21f . This encodes populations ofantibodies formatted as IgGs, originating from first or second roundantibody phage display selections to β-galactosidase or CD229. Ligationswere incubated at 16° C. for 16 hours, purified by spin column andeluted with HPLC grade water.

Ligated DNA was transformed into electro-competent NEB5alpha cells (NewEngland Biolabs, Cat C2989) which generated a library size of 1-4×10⁵clones for each population. DNA was prepared and was co-transfected into100 mls HEK293 cells grown in Freestyle medium as described above using0.3 μg donor DNA (pD6-library) per 10⁶ cells. Cells were co-transfectedwith 0.5 μg each of “AAVS-SBI” TALENs (pZT-AAVS1 L1 TALE-N and pZT-AAVS1R1 TALE, Cat No GE601A-1 System Biosciences).

24 hours after transfection the volume of the bulk culture was doubledand 24 hours later blasticidin (10 μg/ml) was added. Medium wasrefreshed every 3-4 days and after 6 days blasticidin concentration wasincreased to 20 μg/ml.

In order to determine the library size, 250,000 cells were plated in a10 cm petri dish (tissue culture treated) 24 hours after transfectionand were grown in 10% foetal bovine serum (10270-106, Gibco) and 1%Minimal Essential medium non-essential amino acid (MEM_NEAA #11140-035Life Technologies). 10 μg/ml blasticidin was added after another 24hours and medium was changed every 2 days. After 8 days plates werestained with 2% methylene blue (in 50% methanol). Results are shown inTable 3. This shows that libraries of between 5×10⁵ and 9×10⁵ clones(representing 0.5% to 0.9% of transfected cells) were obtained for the 3populations.

TABLE 3 Determination of size of mammalian display libraries formattedas IgG. No. colonies/ No. colonies/ Library size Sample 0.25 × 10⁶ cells10⁶ cells (×10⁵) β-galactosidase Rd1 1337 5348 5.3 β-galactosidase Rd21972 7888 7.9 CD229 Rd2 2175 8700 8.7

The bulk of the population of cells transfected with the outputs from 1or 2 rounds of selection on β-galactosidase were selected in blasticidincontaining medium as described earlier. After 19 days 10-20×10⁶ cellswere labelled and flow sorting carried out as described in example 6.Sorted cells were grown for 17 days and re-analysed by flow cytometry(FIG. 10). This showed that the majority of cells were now doublepositive for IgG expression and binding to β-galactosidase.

Genomic DNA was prepared from the sorted cells and DNA encoding the IgGinsert was isolated by PCR. The IgG-encoding insert was amplified usingKOD polymerase (Merck, cat. no. 71086-3), with annealing temperature of60° C. and employing 30 cycles. Manufacturer provided buffer with 5%DMSO was used with 0.3 μM of primers 2597 (SEQ ID NO: 54) and 2598 (SEQID NO: 47). The product of desired size was gel purified. The gelpurified product was then used for nested PCR using KOD polymerase(Merck, cat. no. 71086-3) in manufacturer's buffer with 5% DMSO using0.3 μM of primer 2625 (SEQ ID NO: 55) in combination with either primer1999 (SEQ ID NO: 56) (for R1 sample), or 2595 (SEQ ID NO: 53) (for 4R1and 5R1), using annealing temperature of 60° C. employing 30 cycles.These nested PCR products were gel purified and subjected to doubledigestion with NheI-HF (NEB, cat. no. R3131S) and XhoI (NEB, cat. no.R0146S) in order to ligate them with similarly double digested pINT3(FIG. 1) for expression of soluble IgG formatted binders. Primersequences are:

2597:  (SEQ ID NO: 85) AGGGGTTTTATGCGATGGAGTT 2598:  (SEQ ID NO: 78)GTTACAGGTGTAGGTCTGGGTG 2625:  (SEQ ID NO: 86) CCTTGGTGCTGGCACTCGA 1999:(SEQ ID NO: 87) AAAAAGCAGGCTACCATGAGGGCCTGGATCTTCTTTCTCC 2595:(SEQ ID NO: 84) GAGGGCTCTGGCAGCTAGC

Example 8. Construction and Selection from a Naïve scFv Library

Schofield et al. [7] describe the construction of a phage displaylibrary (McCafferty library”) wherein antibody genes from theB-lymphocytes of a number of human donors were first cloned into an“intermediate library” before re-cloning into the final functional phagedisplay library. This same intermediated library and the samemethodology was used to generate a new library (IONTAS library) of4×10¹⁰ clones. Plasmid DNA was prepared from this library taking care toensure sufficient representation of the library within the bacterialinoculation. A number of PCR reactions were set up using a total of 2 ugof DNA template. The PCR product was digested with Nco1 and Not 1 gelpurified and ligated as described in Example 6. 9.3 ug of pD6 and 0.93ug of PCR insert were ligated overnight, the ligation reaction cleanedup using phenol chloroform extraction and the DNA electroporated intoDH5alpha cells as described previously [7]. As a result a library of2.4×10⁸ clones was created within the scFv-Fc display vector. DNA wasprepared from this “naïve library” cloned in pD6 and transfected into 1litre of HEK293F cells (Life Technologies) grown in Freestyle medium (asdescribed above). 0.3 ug pD6-library DNA, 0.5 ug of each “AAVS-SBI”TALEN pair. 24 hours after transfection the culture volume was doubledand 48 hour after transfection Blasticidin selection was commenced asdescribed above. The library size was determined by plating aliquots ofthe culture 24 hours after transformation and selecting on blasticidinas described above. A library of 0.9×10⁷ clones was created.

A number of antigens were biotinylated using EZ-Link Sulfo-NHS-LC-Biotinkit (Pierce Cat. No. 21327) according to manufacturer's instructions.Antigens were bovine thyroglobulin (Calbiochem Cat. 609310), humanCD28-Fc chimera (R and D Systems, Cat 342-CD-200) and mouse EphB4-Fcchimera (R and D Systems, Cat. 446-B4-200). Biotinylated β-galactosidase(Rockland Cat. B000-17) was also used.

Transfected cells were selected in blasticidin in liquid culture asdescribed above for 17 days. Cells were harvested, washed and adjustedto 15-20×10⁶ cells per sample. Cells were prepared as described andbiotinylated antigen added to a concentration of 500 nM. Labelling andflow sorting a as described above. Using control cells incubated withonly the phycoerythrin-labelled anti-Fc antibody a “gate” was createdwhich included 0.05% of these cells. Using the same gate for labelledcells between 0.28-0.51% of cells were included (FIG. 11). These werecollected and grown to allow additional rounds of sorting andamplification of scFv genes from the naïve library.

Example 9. Creation of a Cell Line with Multiple “Landing Sites” toCompare Nuclease and Recombinase-Directed Approaches to GenomicIntegration

To allow comparison of integration methods based on either genomiccleavage or recombinase-mediated integration an AAVS-directed targetingvector (pD4) was constructed which introduces an intron with multiple“landing sites” (Example 3). These include an FRT site recognized by Flprecombinase and a pair of a lox2272/loxP sites recognized by Crerecombinase. To allow targeted cleavage, pD4 also includes a sequencefrom GFP for which a TALEN pair have been designed [128] and an I-Sce 1meganuclease site to allow endonuclease-directed integration. Acompatible incoming donor plasmid was constructed (pD5) with appropriaterecognition sites such that nuclease or recombinase directed integrationcauses activation of a promoter-less blasticidin gene and integration anantibody expression cassette.

The organisation of the plasmid and the sequence of pD4 is shown in FIG.12b . An intermediate plasmid pD3 was first created which encompasses aGFP gene under the control of the CMV promoter followed by apuromycin/Thymidine kinase gene fusion under the control of the PGKpromoter (FIG. 12a ). This was created by digesting pBIOCAM1-newNot withSac1 (at the end of the CMV promoter) and BstB1 (between the Neo geneand the poly A site, FIG. 1 a). This removed the neomycin expressioncassette and allows replacement with a synthetic insert encompassing anenhanced Green Fluorescent Protein (EGFP) gene under the control of theCMV promoter. This GFP construct was fused at the C terminus to residues422-461 of a mutated mouse ornithine decarboxylase PEST sequence. ThisPEST sequence is incorporated in plasmid pZsGreen1-DR (Clontech) and hasbeen shown to reduce the half-life of the fused GFP to 1 hour. Acassette encoding a PGK promoter, Puromycin/Thymidine kinase gene fusion(Puro deltaTK) and polyA cassette was excised from the plasmid pFLEXIBLE[129] using Xmn1 and Fse1 and was cloned into Sma1 and Fse1 sitespresent in the original synthetic insert. The resultant plasmid (calledpD3) encodes a CMV-driven GFP gene and a puromycin resistance genedriven from a PGK promoter.

To create the final targeting vector pD4, the CMV promoter was removedand AAVS homology arms were inserted. An 850 bp section of the AAVSlocus was PCR amplified to create an AAVS left homology arm flanked byan EcoR1 at the 5′ end and an Mre1 at the 3′ end. This was cloned intothe ECoR1/Mre1 site of pD3 thereby removing the CMV promoter. An Nsi1site was also incorporated at the 3′ end of this AAVS left homology arm.The neighbouring Mre1 and Nsi1 sites were used to introduce a syntheticfragment fusing an intron to the EGFP gene as shown in FIG. 13. Thesynthetic intron preceding the EGFP gene incorporates:

a FRT recognition site for Flp recombinasea lox 2272 recombination sitean I-Sce1 meganuclease siteGFP TALEN recognition sitea T2A ribosomal stalling sequence [130]

The AAVS right homology arm was generated by PCR to create a Hpa1 andBstZ171 sites at 5′ and 3′ ends. This fragment was cloned into the Hpa1and BstZ171 sites of pD3. The resulting plasmid pD4, encodes a puromycinresistance cassette (“Puro deltaTK”) and can be used to introduce a“landing sites” into the AAVS locus incorporating various nuclease andrecombinase sites for comparison. The sequence of pD4 is shown in FIG.13 (SEQ ID NO: 5, SEQ ID NO: 6 and SEQ ID NO: 7). The AAVS left andright targeting arms are shown in detail in FIG. 3 and are thereforeabbreviated in FIG. 13.

The intron introduced into pD4 contains a TALEN recognition sitesoriginating from GFP [128]. The eGFP directed TALEN pair(eGFP-TALEN-18-Left and eGFP-TALEN-18-right) recognise the sequenceshown below (all sequences and primers are presented in a 5′ to 3′direction) where capitals represent the recognition site of left andright TALENs and the lower case shows the spacer sequence. The righthand TALEN recognises the complement of the sequence shown. The firstbase pair is equivalent of the minus16 position of the sequence relativeto the initiating ATG sequence of GFP (shown bold in the spacer).

(SEQ ID NO: 88) TCCACCGGTCGCCAccatggtgagcaagggCGAGGAGCTGTTCA

The plasmid pD4 also incorporates an I-Sce1 meganuclease site and an FRTsite recognised by Flp recombinase. Finally pD4 incorporates lox 2272and loxP (which are mutually incompatible) flanking the GFP andpuromycin expression cassettes. Incorporation of these same 2 loxP sitesflanking the donor plasmid (pD5 below) affords an opportunity tosubstitute the integrated cassette (including the PGK puro delta TKcassette) replacing it with an incoming cassette driving expression ofblasticidin and antibodies via recombinase mediated cassette exchange.

Creation of a Cell Line by Transfection of pD4.

HEK293F cells were resuspended at 10⁶ cells/ml and DNA:polyethyleneimine (PolyPlus) added at a ratio of 1:2 (w/w). Cells were transfectedwith 0.6m/ml of pD4 and were co-transfected with either the “originalAAVS” TALEN pair or pcDNA3.0 as a control (0.6 μg/ml). pD3 whichexpresses EGFP from the CMV promoter was included in the experiment as atransfection control and showed 35% transfection efficiency. After 24hours transfected cells were plated at 0.5×10⁶ cells/10 cm petri dish(tissue culture treated) and were grown in 10% foetal bovine serum(10270-106, Gibco) and 1% Minimal Essential medium non-essential aminoacid (MEM_NEAA #11140-035 Life Technologies). 5 μg/ml puromycin wasadded after another 24 hours and medium was changed every 2 days. After5 days untransfected cells or cells transfected with pD3 only were dead.After 12 days there were approximately 200 colonies on cells transfectedwith pD4 only and approximately 400 colonies on cells transfected withpD4 and the AAVS TALEN pair.

The puromycin resistant population arising from transfection with pD4and the AAVS TALEN pair was analysed for correct integration. Inaddition a single colony was picked from this population (clone 6F) andcompared with a colony from the puromycin resistant population arisingfrom pD4 transfection in the absence of the AAVS TALEN pair. To identifycorrect integration, PCR primers were designed which hybridise in theAAVS genomic locus beyond the left and right homology arms. These werepaired up with insert specific primers. At the 5′ end the primers were:

AAVS1_HA-L_Nested_Forw1  (SEQ ID NO: 89)GTGCCCTTGCTGTGCCGCCGGAACTCTGCCCTC EGFP_Synthetic_gene_Rev_Assembly(SEQ ID NO: 90) TTCACGTCGCCGTCCAGCTCGAC Purotk_seq_fow2 (SEQ ID NO: 91)TCCATACCGACGATCTGCGAC AAVS1_Right_arm_Junction_PCR_Rev (SEQ ID NO: 77)TCCTGGGATACCCCGAAGAG

FIG. 14 shows that clone 6F and the population are correct at both leftand right ends but the clone picked from the non-AAVS directedpopulation is negative. Thus PCR analysis indicates that the accuracy ofintegration of the donor cassette is greater when directed by AAVS TALENcleavage of genomic DNA.

pD4 introduces a promoter-less, in-frame GFP gene driven from the AAVSpromoter. Flow cytometry of the puromycin resistant population showed anabsence of GFP expression. This failure to express could be due to thecombination of a short half-life (from the murine ornithinedecarboxylase PEST sequence element) combined with reduced expressionarising from the use of the T2A promoter. In fact it was found thataddition of the T2A element in front of a promoterless blasticidinelement (as described for pD2) reduced the number of blasticidinresistant colonies by 4 fold. Despite the absence of GFP expression, theintegration of multiple landing sites still affords an opportunity forcomparison of recombinase-directed versus DNA cleavage directed genomicintegration.

Example 10. Construction of a Vector for Inserting an Antibody Cassette(pD5) into the “Multiple Landing” Site

Following introduction of the “multiple landing site” intron into theAAVS locus, it is possible to introduce an antibody cassette vianuclease-directed or recombinase-directed means. To do this a donorplasmid pD5 was created where the expression cassette is flanked by leftand right homology arms which are equivalent to the sequences flankingthe GFP TALEN cleavage site introduced into pD4. pD5 does not itselfincorporate an intact GFP TALEN recognition site and integration isdriven by homologous recombination. Homology-directed integration of thedonor plasmid will lead to introduction of a blasticidin gene whichlacks a promoter but is preceded by a splice acceptor site that createsan in-frame fusion with the upstream exon from the AAVS locus asdescribed earlier. Integration into the AAVS locus will cause expressionof the promoter-less blasticidin gene. The inserted cassette alsoencodes an IgG formatted antibody heavy and light chains under thecontrol of pEF and CMV promoters respectively as described above. pD5incorporates an I-Sce1 meganuclease site which can lead to cleavage ofthe incoming donor affording an opportunity for NHEJ (see Example 12).An FRT site is also incorporated into the donor plasmid pD5 allowingrecombinase-directed incorporation of the promoter-less blasticidin geneand antibody expression cassettes at the same locus. As discussed aboveCre recombinase will act on the loxP sites in donor and genomic DNA todirect recombinase-mediated cassette exchange.

The sequence of pD5 is shown in FIG. 15. The sequence at the 5′ end ofthe GFP TALEN site is from the AAVS locus. A 267 bp section of the AAVSlocus upstream of the TALEN cleavage site was generated by PCR. Primerswere used which created an EcoR1 and an Mfe 1 site at the 5′ and 3′ endand the product was cloned into the ECoR1 site of pD1-D1.3. The EcoR1site is re-created at the 5′ end. During cloning of the left homologyarm an Nsi1 and Pac1 was also inserted at the 3′ end. A right homologyarm, incorporating approximately 700 bp equivalent to the sequence 3′ ofthe GFP TALEN was created by PCR assembly. PCR primers introduce BstZ171sites at 5′ and at the 3′ end of the assembled fragment and this wascloned into the BstZ171 site of pD1-D1.3. The PCR primers alsointroduced a Hpa1 site at the 5′ end.

A PCR fragment encompassing the intron (which incorporates recognitionsites for GFP TALEN, I-Sce1endonuclease, Flp recombinase and Crerecombinase), a splice acceptor region, a Blasticidin resistance geneand poly A site (described above) was created with Nsi1 site at the 5′end and a Pac1 site at the 3′ end. This was cloned into the Nsi1 andPac1 site of the plasmid described above to create pD5-D1.3 (sequenceshown in FIG. 15 and plasmid structure shown in FIG. 18a ).

Example 11 Comparison of Nuclease-Directed and Flp-Directed Integrationof an Antibody Expression Cassette

The Flp-In system which has previously been used forrecombinase-mediated integration of antibody expression cassettes [18]uses a mutant Flp recombinase (in the plasmid pOG44) which possessesonly 10% of the activity at 37° C. of the native Flp recombinase [19]. Avariant of Flp recombinase (Flpe) with better thermostability andactivity at 37° C. than wild type has been identified [19, 20]. This wasfurther improved by codon optimization to create Flpo [131] encodedwithin plasmid cCAGGS-Flpo (Genebridges Cat. A203). The effect of bothvariants of Flp recombinase (encoded within pOG44 and cCAGGS-Flpo) wascompared. Recombination directed by Cre recombinase was also examined byco-transfecting cells with a plasmid which encodes Cre recombinase [132](pCAGGS-Cre, Genebridges Cat. A204). In each vector the recombinase isexpressed under the control of the chicken-B-actin promoter and a CMVimmediate early enhancer. An SV40 Large T nuclear localization sequenceis used for nuclear localisation [20]. In the original vectors(cCAGGS-Flpo and pCAGGS-Cre) recombinase expression was linked to apuromycin resistance gene by an internal ribosomal entry site (IRES)which was removed using standard molecular biology techniques.

An experiment was carried out to compare the efficiencies of genomiccleavage-directed versus recombinase directed integration of an antibodycassette. The outcome was assessed in 2 ways:

1. Measuring the number of blasticidin-resistant colonies arising fromintegration of a promoter-less blasticidin gene2. Assessing the extent of antibody expression achieved by the differentapproaches.

As described in Example 9, the recognition sites for Cre recombinase(lox2272 and loxP) and Flp recombinase (FRT) were previously integratedinto the AAVS locus within clone 6F. In addition recognition sites for aGFP TALEN pair and for the meganuclease I-Sce1 are also present withinthe same intron. The donor plasmid pD5-D1.3 carries the same recognitionsites (apart from GFP TALEN) within an intron upstream of apromoter-less blasticidin gene. Correct integration will lead toactivation of the blasticidin gene. pD5-D1.3 also encodes an IgGformatted D1.3 antibody gene which will be expressed on the cellsurface.

Co-transfection of pD5-D1.3 with pOG44 or pCAGGS-Flpo (encoding 2variants of Flp recombinase) should result in integration of the entirepD5 plasmid into the FRT site of clone 6F. The donor plasmid pD5-D1.3also has lox2272 site within the synthetic intron upstream of theblasticidin gene and a loxP site at the end of the antibody expressioncassette. Under the action of Cre recombinase expressed from pCAGGS-Cre,recombinase-mediated cassette exchange should result in the integrationof the blasticidin and antibody expression cassettes into the lox2722and loxP sites within clone 6F.

The efficiency of vector integration using recombinase-directedapproaches with genomic cleavage-directed approaches was compared usinga pair of TALENs (eGFP-TALEN-18-Left and eGFP-TALEN-18-right) directedtowards a sequence from GFP (Reyon et al., 2012). In the case of GFPTALENs the element between the left and right homology arms will beintegrated following genomic cleavage by TALENs.

To allow comparison with I-Sce1 meganuclease a codon optimised geneencoding I-Sce1 was constructed (FIG. 16). This gene has an N terminalHA epitope tag/SV40 nuclear localisation signal (NLS) at the N terminusand is flanked by Nco1 and Xba1 sites at the 5′ and 3′ termini. The genewas cloned into the vector pSF-CMV-F1-Pac1 (Oxford Genetics OG111) whereexpression is driven from the CMV promoter.

Transfections were carried out using the clone 6F with a correctlyintegrated “multiple landing site”. Cells were suspended at 10⁶/ml andtransfected with 50 ng of pD5-D1.3 donor plasmid/10⁶ cells along withenzyme encoding plasmids (Table 4A).

After 24 hours transfected cells were plated at 0.5×10⁶ cells/10 cmpetri dish (tissue culture treated) and were grown in 10% foetal bovineserum (10270-106, Gibco) and 1% Minimal Essential medium non-essentialamino acid (MEM_NEAA #11140-035 Life Technologies). 5 ug/ml blasticidinwas added after another 24 hours and medium was changed every 2 days.After 12 days plates were stained with 2% methylene blue (in 50%methanol) and colony numbers counted (Table 2). In a direct comparisonbetween Flp recombinase, Cre recombinase and TALEN, the greatest numberof colonies were obtained through use of the GFP TALEN where there was a9-fold increase compared with “donor only” (Table 4A). It also appearsthat the use of the optimised Flpo gene actually resulted in a reductionof the number of blasticidin resistant colonies compared to “donor only”control, presumably through toxicity of the enhanced activity Flprecombinase. There was also an increase in colony number usingpCAGGS-Cre compared to the donor only control.

A second experiment was carried out comparing GFP TALEN with bothenhanced Flp (from cCAGGS-Flpo) as well as the low activity Flp enzymeencoded within pOG44 from the Flp-In system (as used by Zhou et al. [17,18, U.S. Pat. No. 7,884,054]. These were compared with GFP TALEN and Crerecombinase (Table 4B). Cells were transfected with the amounts DNAshown per million cells. 0.25×10⁶ cells were plated out and the numberof blasticidin resistant colonies determined as described above. Cellswere also selected for blasticidin resistance in liquid culture for 30days before determining the proportion of cells which expressed surfaceIgG (as described above). Table 4B shows that the TALEN was superior tothe other approaches in terms of number of resistant colonies. Again theuse of optimised Flp within cCAGGS-Flpo actually caused a reduction innumber of blasticidin resistant colonies compared to “donor only”controls. Cre recombinase again led to an increase in blasticidin colonycount compared with control whereas the Flp gene within pOG44 showedonly a marginal increase compared with control.

TABLE 4 Comparison of TALE nuclease-directed and recombinase-directedintegration approaches. A. pD5-D1.3 donor Blasticidin resistant coloniesEnzyme plasmid Amount (ng/10⁶ cells) (colonies/10⁶ cells) GFP TALEN pair 0.575 μg 50 152 (304) each pCAGGS-F1p₀ 1.15 μg 50 1 (2) pCAGGS-Cre 1.15μg 50 57 (114) control ( pCDNA3.0) 1.15 μg 50 17 (34) B. Blasticidincolonies Sample pD5-D1.3 per 10⁶ cells 2 μg each per 10⁶ DonorBlasticidin (percentage blasticidin Percentage positive in cells μg/10⁶cells colonies resistant) flow GFP TALEN pair 0.6 270 1080 (1.1%) 95.6cCAGGS-F1p₀ 0.6 35   140 (0.14% Too few cells pOG44 0.6 96   384 (0.38%)6.4 pCAGGS-Cre 0.6 180   720 (0.72%) 4 Control 0.6 81   324 (0.32%) 37.3(PCDNA3.0) C. Sample 2 μg each per 10⁶ pD5-D1.3 Donor cells μg/10⁶ cellsBlasticidin colonies Percentage positive in flow GFP TALEN 2 210 95.3GFP TALEN 6 120 73.6 cCAGGS-F1p₀ 2 62 Too few cells cCAGGS-F1p₀ 6 41 Toofew cells pOG44 2 178 35.6 pOG44 6 63 58.7 pCAGGS-Cre 2 84 40.9pCAGGS-Cre 6 52 Too few cells Control 2 340 65.6 (PCDNA3.0) Control 6 8255.5 (PCDNA3.0)

With the addition of more donor DNA (Table 4C) there was an increase incolony numbers at intermediate levels (2 μg/million cells) and a declineat higher levels across the board (6 μg/million cells). None of theother samples achieved levels of antibody display seen with the GFPTALEN directed integration.

Cells were also selected for blasticidin resistance in liquid cultureand the cells stained for antibody expression as described above.TALEN-directed integration gave a significantly higher proportion ofantibody-positive cells compared with the other approaches. Cellstransfected with cCAGGS-Flpo and with high concentrations of pCAGGS-Crewere not healthy and there were insufficient numbers to carry out flowcytometry.

The comparison was extended to included I-Sce endonuclease. A syntheticgene encoding I-Sce1 was synthesised (FIG. 16) and cloned into theNco1/Xba 1 site of pSF-CMV-f1-Pac1 (Oxford Genetics). Cells weresuspended at 10⁶/ml and transfected for each ml of cells (10⁶ cells/ml)with 300 ng of pD5-D1.3 donor plasmid along with plasmids encodingenzymes (1 ug/10⁶ cells). Next day 0.05 ml of cells were plated andselected in blasticidin and stained after 14 days as described. Table 5shows that the highest number of blasticidin resistant colonies camefrom I-Sce1 meganuclease followed by the eGFP TALEN pair. Both Cre andFlp recombinase (encoded within pOG44) gave numbers slightly higher thanthe “donor only” control. As before transfection with the Flpe encodingplasmids actually reduced colony numbers compared to “donor only”.

TABLE 5 Comparison of meganuclease-directed and recombinase-directedintegration approaches Blasticidin colonies pD5-D1.3 per 10⁶ cellsPercentage Sample Donor (percentage Percentage positive in 2 μg each perμg/10⁶ Blasticidin blasticidin positive in flow 10⁶ cells cells coloniesresistant) flow d7 d13 GFP TALEN 0.6 90 1800 27.6  55% pair I-Sce1 1503000 29.9  47% meganuclease pOG44 0.6 60 1200  2.79 6.5% pCAGGS-Cre 0.656 1120  2.95 6.6% cCAGGS-F1p₀ 0.6 4 80 5.9 (low cell Too few nos) cellsControl 40 800  3.3 4.9% (PCDNA3.0) (SBI AAVS 251 (x2) 10040 (1%) ND NDinto WT HEKs

After transfection the bulk of cells were selected for blasticidinresistance in liquid culture and after 7 and 13 days were stained withan anti-Fc phycoerythrin labelled antibody as described above. FIG. 17(summarised in Table 5) shows significantly higher antibody expressionwas achieved for cells transfected with I-Sce1 endonuclease and eGFPTALEN (47% and 55% respectively) compared to “donor only” (4.9%). Incontrast the percentage of antibody positive cells when cells wereco-transfected with plasmids encoding Flp recombinase (pOG44) or Crerecombinase were 6.6 and 6.5% respectively. The proportion of antibodypositive cells continues to increase with continued selection inblasticidin and achieves 85-90% antibody positive in the case of theI-Sce1 and EGFP TALEN transfected samples when assayed on day 19. Thusmeganucleases provide an alternative approach to effectnuclease-directed integration of antibody-encoding transgenes.

Example 12. Nuclease-Directed Integration of an Antibody Cassette canOccur by Both Homologous Recombination and NHEJ

The efficiency of integration of transgenes into cellular DNA can beenhanced by the introduction of double stranded breaks (DSBs).Endogenous DNA repair mechanisms in eukaryotic cells include homologousrecombination non-homologous end joining (NHEJ) and variants of these.All provide a means for introducing genes encoding binders within alibrary. Homologous recombination provides a precise join between theregions of homology and the inserted transgene but require the provisionof regions of homology in the donor plasmid. DNA for homologousrecombination can be provided as linear or circular DNA. With NHEJ theends of DNA are directly re-ligated without the need for a homologoustemplate. This approach to DNA repair is less accurate and can lead toinsertions or deletions. NHEJ nonetheless provides a simple means ofintegrating in-frame exons into intron or allows integration ofpromoter:gene cassettes into the genome. Use of non-homologous methodsallows the use of donor vectors which lack homology arms therebysimplifying the construction of donor DNA.

Clone 6F has GFP TALEN and I-Sce1 nuclease recognition sites integratedinto the genome and these will be cleaved when these nucleases areprovided. The donor vector pD5 does not have a GFP TALE nucleaserecognition site but has homology arms flanking the cleavage site and sois expected to integrate by homologous recombination only. Cleavage ofgenomic DNA at the neighbouring I-Sce1 meganuclease will also lead tointegration of the pD5 elements by homologous recombination. pD5 howeveralso has an I-Sce1 meganuclease site which can be cleaved in vivo whenI-Sce1 is provided. This will create a linear DNA product which canpotentially be integrated by NHEJ. As described earlier there may evenbe efficiency advantages using in vivo cleavage of donor DNA when NHEJis used.

FIG. 18a represents the incoming pD5-D1.3 donor DNA and FIG. 18brepresents the genomic locus of clone 6F cells incorporating the“multiple landing” site. FIG. 18c represents the consequence ofhomologous recombination between pD5-D1.3 (FIG. 18a ) and the multiplelanding site of clone 6F (FIG. 18b ). FIG. 18d in contrast representsthe consequence of NHEJ. In this case extra DNA from the backbone of theincoming plasmid is incorporated (represented by a double arrow). Flpmediated recombination at the “multiple landing” site will lead to asimilar product. In order to determine which route is being used withthe samples described in Example 11 (shown in FIG. 17) genomic DNA wasprepared from the blasticidin selected population as described before. Areverse PCR primer (J44) was designed which hybridizes to the integratedPGK promoter. This was used in conjunction with either J48 whichhybridises at the end of the IgG protein. Primers J44 and J48 weredesigned to reveal homologous recombination producing a band of 1928 bpwhen I-Sce1 is responsible for integration (indicated by arrow in FIG.18e ). (Potentially a band of 5131 bp could be produced by this primerpair when NHEJ has occurred but this longer product was not visible inthe genomic PCRs of this experiment.)

Primer J46 was designed to hybridise within the β-lactamase gene withinthe vector backbone. Primers J44 and J46 are anticipated to produce aband of 1800 bp when NHEJ has occurred. A similar sized band is expectedwhere Flp recombinase has led to recombinase-mediated integration.

J44: (SEQ ID NO: 92) AAAAGCGCCTCCCCTACCCGGTAGAAT J46:  (SEQ ID NO: 93)GGCGACACGGAAATGTTGAATACTCAT J48: (SEQ ID NO: 94)CACTACACCCAGAAGTCCCTGAGCCTG

FIG. 18e clearly reveals that homologous recombination occurs only withthe samples treated with GFP TALEN and I-Sce1 meganuclease ((i and iicompared with iii and iv). In contrast NHEJ only occurs when cleavage iseffected by I-Sce1 meganuclease (FIG. 18e v.) but not GFP TALEN (FIG.18e vi). As expected a similar size band is found in the sample treatedwith Flp recombinase (FIG. 18e vii). Thus this experiment revealsnuclease-directed integration of an antibody cassette can occur by bothhomologous recombination and NHEJ.

Example 13. Generation of Secreted and Membrane Bound Antibody Fragmentsfrom the Same Cell

As described above, mammalian display vectors pD2 and pD5 wereconstructed with an exon encoding a transmembrane domain flanked by twoROX recognition sites recognised by Dre recombinase [88]. In order todetermine whether it was possible to convert from a membrane bound formto a secreted form, the blasticidin resistant population arising fromtransfection with pD2-D1.3/AAVS TALEN pair was re-transfected with theplasmid encoding Dre recombinase (pCAGGs-Dre). This was based on theplasmid pCAGGs-Dre-IRES puro [88] which drives the Dre recombinase genefrom a CAGGs promoter (GeneBridges A205). The puromycin resistance genewas removed using standard molecular biology techniques. After 22 daysof blasticidin selection, cells were set up at 0.5×10⁶ cells/ml andtransfected as described earlier with 0.5 μg pCAGGs-Dre per 10⁶ cells.After 6 days supernatants were collected, antibody purified usingprotein A and samples run on an SDS-PAGE gel and stained with Coomassieblue. FIG. 19a shows that secreted antibody was found in the supernatanteven without transfection with the Dre recombinase gene. This may arisefrom alternative splicing where the exon encoding the transmembranedomain is skipped. Alternatively antibody in the culture supernatantcould arise from cleavage of the membrane bound antibody. Transfectionof Dre recombinase increased the level of secreted antibody (FIG. 19a ).

Production of secreted scFv-Fc fusion was also demonstrated in theexperiment describe in example 7 (FIG. 9h ). Antibody scFv populationsselected by 1 round of phage display on β-galactosidase were introducedinto the pD6 vector and integrate into the AAVS locus of HEK293 cellsusing the AAVS TALEN. Antigen binding cells were sorted by flow sortingand selected cells were grown for 7 days post-sorting without a changeof medium to allow antibody to accumulate. ELISA plates were coated witheither β-galactosidase (10 ug/ml) or BSA (10 ug/ml) overnight. Culturesupernatants from 7 day cultures were mixed with a 50% volume of 6%Marvel-PBS and the sample tested in triplicate. A 1/10 dilution was alsotested. Detection of bound scFv-Fc fusion was performed using anti-HumanIgG-Eu (Perkin Elmer Cat 1244-330). FIG. 19b shows that antibody bindingcan be detected directly from culture supernatants either neat or with a1/10 dilution. This illustrates that both surface display and antibodysecretion can be achieved within the same cells without additionalsteps. It will be possible to derive secreted antibody directly from theselected cells either following single cell cloning or using a sortedpopulation as shown here generate a polyclonal antibody mix.

Example 14. A Simple Method for Conversion of scFvs to IgG or Fab Format

A novel method was invented to effect the conversion of antibodiesformatted as scFv to an IgG format as described in Example 7. Thisconversion is a necessary process during antibody drug discoveryprojects employing scFv antibody phage display libraries where an IgG orFab formatted antibody is required as the final format. Current methodsare tedious and involve individual cloning of the variable heavy (V_(H))and variable light (V_(L)) chains into suitable expression vectors.Furthermore conversion of a population of scFvs “en masse” is notpossible because the link between the V_(H) and V_(L) chains is lost.This is a problem because both the V_(H) and V_(L) chains contribute toantigen binding specificity. The current inability to easily convertpopulations of scFv to Ig or Fab format limits the ability to screenlarge numbers of antibodies in the final format they will be used in theclinic. The ability to screen recombinant antibodies in Ig or Fab formatfor target binding, cell reporter screens and biophysical properties andfunction including aggregation state is a necessary step to choosecandidate antibody drugs as clinical candidates. The greater number ofantibodies tested at this stage, in IgG or Fab format, the greater thechance of selecting the best antibody drug candidate.

Described here is a method to convert single chain antibody (scFv)populations to immunoglobulin (Ig) or Fab format in such a way that theoriginal variable heavy (V_(H)) and variable light (V_(L)) chainpairings are maintained. The method allows one to convert monoclonal,oligoclonal or polyclonal scFvs simultaneously to Ig or Fab format.Preferably, the method proceeds via the generation of a non-replicative“mini-circle” DNA. Preferably the complete conversion process entails asingle transformation of bacteria such as E. coli to generate apopulation of bacterial colonies each harbouring a plasmid encoding aunique Ig or Fab formatted recombinant antibody. This is distinct fromalternative methods requiring two separate cloning and transformationsteps [117].

More broadly this aspect of the invention relates to a method ofconverting a genetic construct with 3 linked genetic elements A, B and C(represented by the V_(H), linker and V_(L) respectively in the case ofan scFv) to a format where the order of the flanking elements (A and C)are reversed, in a single cloning step. The intermediate element couldbe retained but most usefully the method permits the replacement of thisintermediate element by a new element D (to give C-D-A). In the exampleof conversion of a scFv to an IgG or Fab then C is an antibody V_(L)domain and A is a V_(H) domain. In this example element D encapsulate alight chain constant domain, poly A site, promoter and leader sequencefused to the V_(H) (element A). In the process the product (C-D-A) isre-cloned allowing the flanking sequences to also be changed. In thescFv to IgG conversion example the V_(L) element is preceded by apromoter and leader sequence and the V_(H) is followed by aC_(H1)-C_(H2)-C_(H3) domain in the case of IgG formatted antibodies andby a C_(H1) domain in the case of Fab formatted antibodies. The methodcould be applied more broadly where elements A and C (using abovenomenclature) could represent other genetic elements, e.g., inconstruction of proteins with circularly permutation where the originalN and C termini are fused and novel internal termini are engineered.

FIG. 21 illustrates the conversion process schematically using scFv toIgG conversion as an example. A DNA insert (a) encoding the antibody VHand VL domains is ligated with DNA fragment (b) encoding a constantlight (CL) chain, a polyadenylation sequence (pA), a cytomegalovirus(CMV) promoter and a signal peptide (SigP). DNA fragment (b) could alsoencode any promoter in place of the CMV promoter. Also the pA-CMVcassette could be replaced by an internal ribosomal entry sites (IRES)[119] or a 2A type “self-cleaving” small peptide [130, 133]. The joiningof DNA molecules (a) and (b) to create a non-replicative DNA“mini-circle” (c) is facilitated by a “sticky-end” ligation. In FIG. 21,Nco1 and Not1 sites are employed because these were used in the creationof the McCafferty phage display library [7] however any suitablerestriction sites could be used to create the non-replicative“mini-circle” c. After ligation, the “mini-circle” c is linearized withrestriction enzymes NheI and XhoI, the recognition sites of which flankthe linker between the V_(H) and V_(L) domains. NheI and XhoI werechosen to illustrate this invention because they were used in thecreation of the McCafferty phage display library [7], however anysuitable restriction sites could be used.

Linearised product d is then purified and ligated with the digestedvector (e). The vector (e) includes a CMV or pEF promoter and signalsequence upstream of the NheI site and encodes the antibody constantheavy (C_(H)) domains 1 to 3 downstream of the XhoI site. The vectorwould also encode a bacterial origin or replication and antibioticresistance marker (not shown) to enable selection and replication of theresultant plasmid DNA in bacteria. The product of ligation of insert (d)with vector (e) would result in plasmid f, which can be used totransform bacteria and growth with a suitable selectable marker wouldallow the production and purification of plasmid DNA by standardmethods. Purified plasmid f can be introduced into mammalian cells [134]for heterologous Ig antibody expression. Alternatively DNA encodingCH₁₋₃ in vector (e), could be replaced with DNA encoding a single C_(H1)domain for Fab expression.

In the detailed method description below used to illustrate thisinvention, the insert b contains either a CMV promoter or a P2A peptidewhich enables expression of the separate antibody light and heavy chainsfrom a single messenger RNA (mRNA). The method is non-obvious and wasrefined after several experimental attempts. For example, initiallylinearisation of the DNA “mini-circle” (c) was attempted by PCR. Howeverthis resulted in the amplification of homo-dimer side-products,resulting in a low yield of the desired product (d). In contrast, directdigestion of the DNA “mini-circle” (c) provided sufficient material (d)to allow the method to be successfully implemented. Secondly, in anattempt to prevent undesired homo-dimer product, insert (a) wasinitially dephosphorylated. However, this required careful control toprevent “end” digestion resulting in product lacking the desired“sticky-ends” for ligation. The optimal method does not includedephosphorylation to maximise the proportion of ligation competentproduct. Lastly, careful control of the ratios used in the ligation ofDNA inserts (a) and (b) was required to maximise the yield of the DNA“mini-circle” (c).

1. Preparation of PCR scFv InsertsFrom a bacterial glycerol stock, harbouring plasmid DNA encoding scFvscrape into 50 ul of water. Dilute this 1 in 10. Use 5 ul of this forPCR reaction containing forward primer pSANG10pelB (CGCTGCCCAGCCGGCCATGGSEQ ID NO: 95) (2.5 μl, 5 reverse primer 2097(GATGGTGATGATGATGTGCGGATGCG SEQ ID NO: 96), (2.5 μl, 5 μM), 10×KODbuffer (KOD hot start kit from Merck, 71086-4), dNTPs (5 μl, 2 mM),MgSO4 (2 μl, 25 mM), KOD hot start polymerase (2.5 units) in a totalvolume of 50 μl. Cycling conditions were 94° C. for 2 min then 25 cyclesof 94° C. 30 sec, 54° C. for 30 sec then 72° C. for 1 min. PCR clean upwas performed by spin column (Qiagen or Fermentas) and the PCR reactionseluted in 90 FIG. 22a shows 1 μl of PCR reaction loaded on a 1% agaroseTBE gel. Purified scFv DNA (80 μl, 8 μg) was digested by the addition ofbuffer 4 (New England Biolabs), BSA (0.1 mg/ml) and 40 units of NcoI-HFand NotI-HF in a total volume of 100 μl and incubated for 2 hours at 37°C. Inserts were purified with a Qiagen PCR cleanup kit, eluted in 30 μland the DNA concentration measured by measuring the absorbance at 260 nMwith a nanodrop spectrophotometer (Thermo).

2. Ligation of DNA Inserts (FIGS. 21 a and b)

A ligation reaction is performed to produce the DNA “mini-circle” (FIG.21c ). The ligation reaction contains insert b (125 ng), scFv insert a(FIG. 21) (125 ng), 10× ligation buffer (Roche T4 DNA ligase kit, 1.5ul), T4 DNA ligase (1 unit) in a total volume of 15 μl. Incubate 1-2 hr21° C. Water (35 μl) was added to the ligation mix and purified with aQiagen PCR cleanup kit and eluted in 30 μl3. Digestion of DNA “Mini-Circle” (FIG. 21c ) with Xho1/Nhe1Purified ligation reaction (28 μl) is digested by the addition of buffer4 (New England Biolabs, 3.5 μl), BSA (0.1 mg/ml) and 10 units of NcoI-HFand NotI-HF in a total volume of 35 μl and incubated for 2 hours at 37°C. This is then purified by separation on a 1% agarose TBE (FIG. 22b ).Alternatively FIG. 22c shows a linearised “mini-circle” containing a P2Asequence in place of a CMV promoter. DNA band at 2.6 kb (FIG. 22b ) isexcised and purified with Qiagen gel extraction kit and eluted in 30 μl.4. Ligation of Linearised DNA “Mini-Circle” d with pINT3 (XhoI/NheI Cut)Vector and Transformation of E. coli DH5α.A standard ligation was set-up with pINT3 cut vector (50 ng), linearised“mini-circle” d (20 ng), 10× ligase buffer (Roche, 1.5 μl) and 1 unit ofT4 DNA ligase (NEB) in a final volume of 15 μl. Incubation was at 21° C.for 2 hrs. Transformation of E. coli DH5alpha chemically competentcells, subcloning efficiency, (Invitrogen, cat. 18265017) was accordingto the manufacturer's instructions. 80 μl of chemically competent DH5acells were added to 6 μl ligation mix, placed on ice for 1 hour and heatshocked at 42° C. 1 min, ice for 2 min and then transferred to a 14 mlpolypropylene tube containing 900 μl SOC media and incubated at 37° C.for 1 hour and plated on LB amp plates.

Example 15: Construction of Large Display Libraries in Mammalian Cellsby Nuclease-Directed Integration Using Flow Electroporation

Electroporation is an efficient way of introducing DNA, RNA and proteininto cells and electroporation flow systems allow for efficientintroduction of DNA into large numbers of mammalian cells. For examplethe “MaxCyte STX Scalable Transfection System” (Maxcyte) permits theelectroporation of 10¹⁰ cells within 30 mins, creating the potential fortransfecting up to 10¹¹ cells in a day. Cells and DNA are mixed andpassed from a reservoir to an electroporation chamber, electroporated,pumped out and the process repeated with a fresh aliquot of cells andDNA. The same method can be applied for introduction of DNA, RNA,protein or mixtures thereof into cultured cells (e.g., human HEK293cells or Jurkat cells) or primary cells e.g. human lymphocytes [135].Flow electroporation has been used to efficiently introduce DNA, RNA andprotein into a large number of primary and cultured cells.

Here we exemplify the use of such a system to introduce donor DNA,encoding antibody genes, by co-transfecting with DNA encoding a pair ofTALE nucleases targeted to the human AAVS locus of human HEK293 cellsand Jurkat cells.

The distribution of the 2 different antibody specificities wasdetermined by flow cytometry using fluorescently-labelled antigen. Thegeneration of antibodies recognising the FGF receptors FGFR1 or FGFR2has been described previously [105]. Clones α-FGFR1_A and α-FGFR2_A(described therein) were cloned into pD6 as described in example 6. Inaddition a population of scFv antibodies selected from the “McCafferty”phage display library [7] using one round of phage display onβ-galactosidase (β-gal) were also cloned into this vector (as describedin example 6).

HEK293 cells were centrifuged and re-suspended in a final volume of 10⁸cells/ml in the manufacturer's electroporation buffer (MaxcyteElectroporation buffer, Thermo Fisher Scientific Cat. No. NC0856428)).An aliquot of 4×10⁷ cells (0.4 ml) was added to the electroporationcuvette with 100 μg DNA (i.e., 2.5 μg/10⁶ cells). The amounts of thedifferent components used are shown below. Donor DNA encoding antibodiesα-FGFR1_A and α-FGFR2_A was provided as an equimolar mix with the totalamount per 10⁶ cells shown in Table 6 below. DNA encoding AAVS-SBITALENs (pZT-AAVS1 L1 and pZT-AAVS R1 Systems Bioscience Cat. No.GE601A-1) was used as an equimolar mix with the total amount per 10⁶cells shown in Table 6 below. In samples without added TALENs, the inputDNA was brought to 2.5 μg/10⁶ cells using control plasmid pcDNA3.0.

The percentage transfection efficiency was calculated by counting thenumber of blasticidin colonies achieved for a given input of totalcells. The fold difference compared to negative controls (i.e., no TALENDNA added) is shown in brackets. Finally, the number of transformedcolonies achievable by running a full cycle of the Maxcyte system,involving electroporation of 10¹⁰ cells is calculated in the lastcolumn. This represents a single cycle of approximately 30 minutes,giving the potential to run multiple cycles in a day. Thus, the dailyoutput could be 5-10-fold higher. Large scale fermentation and culturesystems such as Wavebag system (GE Healthcare) or the Celltainer system(Celltainer Biotech) can be used to generate cells for transfection andcan be used to cultivate the resulting libraries.

TABLE 6 Electroporation of HEK293 cells αFGFR1_A/αFGFR2_A ng TALENDNA/10⁶ No clones per Sample ng donor DNA/10⁶ cells cells % transfection10¹⁰ cells  1  580 1920 5.1 (51×) 5.1 × 10⁸  1b  580  640 3.3 (33×) 3.3× 10⁸  2  580 — 0.1 —  0.1 × 10⁸  3  194 1920 2.7 (89×) 2.7 × 10⁸  4 194 — 0.03 0.03 × 10⁸   5 1185 1315 5.8 (25×) 5.8 × 10⁸  6 1185 — 0.230.23 × 10⁸   7 1825  675 6.1 (11×) 6.1 × 10⁸  8 1825 — 0.57 0.57 × 10⁸  9 580 (FGFR1 alone) 1920 5.3  5.3 × 10⁸ 10 580 (FGFR2 alone) 1920 5.3 5.3 × 10⁸ β-galactosidase donor DNA/10⁶ ng TALEN % transfection Noclones per Sample cells DNA/10⁶ cells 10¹⁰ cells 11 580 1920 4.5 4.5 ×10⁸ 12 580 — 0.21 0.21 × 10⁸  13 1185 1315 5.5 5.5 × 10⁸ 14 1185 — 0.210.21 × 10⁸ 

This example demonstrates that it is possible to make very largelibraries of cells with integrated antibody cassettes. The transfectionefficiency ranged from 2.7 to 6.1%. In the case of the β-galactosidaseselected population (sample 13) a library of 5.5×10⁸ clones can becreated in a single flow electroporation session. With more than onesession in a day, a library of 2-5×10⁹ clones can be generated.

After 13 days of blasticidin selection (10 μg/ml), cells were labelledwith phycoerythrin labelled anti-Fc antibody (Biolegend, Cat. No.409304) as described earlier. Of the antibody population selected onβ-galactosidase, 34-36% of cells were positive for Fc expression and11-13% were positive for binding of Dyelight-633-labelled antigen at 10nM concentration.

Where FGFR binding clones were used, 98-99% of cells were positive forFc expression. Use of a mixture of α-FGFR1_A and α-FGFR2_A antibodiesaffords an opportunity to examine the proportion of cells containingmultiple integration events. For an individual cell with acorrectly-integrated cassette (e.g., α-FGFR1_A) there is approximately a50:50 chance that a second integration will be of the alternativespecificity (i.e., α-FGFR2_A). If there are frequent multipleintegrations, then the proportion of double-positive clones will behigh, however, the proportion of double-positive clones was not found tobe high, illustrating the fidelity of the nuclease-directed libraryintegration system in generating one antibody gene/per cell. The abilityof the surface displayed anti-FGFR antibodies to specifically bind theirappropriate antigen was confirmed. Expression of antigen was from aplasmid pTT3DestrCD4(d3+4)−His10 [134] encoding mouse Fgfrl ectodomain(ENSMUSP00000063808). This was used to transfect HEK293 suspension cellsand secreted Fgfrl-rCd4-His10 purified by immobilised metal affinitychromatography as described previously [134]. Mouse Fgfr2 ectodomain wasPCR amplified from IMAGE clone 9088089 using primers:

2423  (SEQ ID NO: 97) (TTTTTTCCATGGGCCGGCCCTCCTTCAGTTTAGTTGAG) and 2437(SEQ ID NO: 98) (TTTTTTGCGGCCGCGGAAGCCGTGATCTCCTTCTCTCTC),digested with NcoI/NotI and cloned into expression plasmid pBIOCAM5[126]. Fgfr2-Fc was expressed by transient transfection of HEK293 cellsas described previously [134] and purified by affinity chromatography.

Transfected populations were probed for dual binding using both of thelabelled antigens and the proportion of double positives was low. Inthis experiment the optimal balance of library size (2.7×10⁸ clones/perMaxcyte session) with low percentage of double positives (3.5%) wasfound using 197 ng donor DNA per 10⁶ cells (FIG. 23).

This proportion of double positives could represent mis-incorporation ofa second antibody cassette but given the efficiency of nuclease-directedintegration of the library it is also possible that both alleles (theAAVS locus in this example) could be targeted with incoming binders in aproportion of cells. The presence of two different antibody genes withina cell in itself does not prevent the isolation of binders or theirencoding genes but this can be circumvented by first modifying thetarget cell at a single locus to introduce a single nuclease targetingsite, e.g., a pre-integrated Sce1 meganuclease site as demonstrated inexample 9.

Example 16. Recovery of Genes Encoding Binders from Sorted LibraryPopulations

Phage display selections were carried on β-galactosidase (as describedin example 6) and antibody populations from 1-2 rounds of selection werecloned into vector pD6 and introduced into the AAVS locus of HEK293cells as described in example 6. β-galactosidase was labelled usingLightning Link Dyelight-633 (Innova Bioscience Cat. No 325-0010)according to manufacturer's instructions. Transfected cell populationswere selected for 25 days in blasticidin (10m/ml) and were labelled with10 nM Dyelight-633 labelled β-galactosidase and phycoerythrin-labelledanti-Fc (Biolegend Cat. No. 409304). Cells were incubated withantibodies for 30 minutes at 4° C., washed twice in PBS/0.1% BSA,re-suspended in PBS/0.1% BSA and double-positive cells sorted using aflow sorter.

Sorted cells were expanded and a second round of sorting was carried outusing 10 nM antigen. Cells were grown and either genomic DNA or mRNA wasisolated from the sorted, selected populations.

Where binders encompassing different chains (e.g., IgG formattedantibodies) are present on the same genomic sequence (e.g., byintroduction on the same plasmid) but are transcribed into differentmRNAs it may be optimal to recover the separate genes encoding themultimeric binder by amplification from genomic DNA. As an alternative,binders encompassing multiple protein chains can be encoded on the samemRNA through the use of “internal ribosome entry sequence” (IRES)elements or sequences such as viral P2A or T2A sequences that promotetranslational stalling/protein cleavage [133, 136]. In this case and inthe case of binders encoded on a single protein chain it will also bepossible to isolate the encoded genes from mRNA.

Genomic DNA was prepared using the “DNeasy blood and tissue kit” (QIAGENCat. No. 69504). mRNA was prepared using an “Isolate II RNA mini kit”(Bioline Bio-52072). For amplification from genomic DNA a PCR reactionwas set up using Phusion polymerase with the “2×Phusion GC” mixaccording to manufacturer's instructions. Primers which flank the Nco1and Not1 cloning sites, e.g., primers 2622 (GAACAGGAACACGGAAGGTC) (SEQID NO: 99) and 2623 (TAAAGTAGGCGGTCTTGAGACG) (SEQ ID NO: 82) were usedto amplify the antibody cassette (98° C. 10 secs, 58° C. 20 secs, 72° C.90 secs for 35 cycles.

Genes encoding selected scFv genes were amplified from mRNA. Total RNAwas isolated from the sorted cells using the “Isolate II RNA mini kit”(Bioline Cat No Bio-52072). cDNA was synthesized from 2 μg RNA usingSuperscript II reverse transcriptase (Life Technologies, Cat no180064-022). The selected scFv genes were then amplified from the cDNAby PCR using KOD Hot Start DNA polymerase (Merck Millipore Cat. No.71086-3) using primers which flank the Nco1 and Not1 cloning sites. Inthis case, primers:

41679  (SEQ ID NO: 100)  ATGAGTTGGAGCTGTATCATCC and  2621 (SEQ ID NO: 101) GCATTCCACGGCGGCCGCwere used to amplify the antibody cassette (95° C. for 20 seconds, 60°C. for 10 seconds, 70° C. for 15 seconds, for 25 cycles). PCR productswere digested with Nco1 and Not 1 before cloning into the Nco/Not1 siteof bacterial antibody expression vector pSANG10. Construction ofpSANG10, methods for bacterial expression and screening by ELISA aredescribed in Martin et al 2006 [127].

ELISA screening of the population from 1 round of selection by phagedisplay revealed that 0/90 of the clones were positive. In contrast,when this same population was further subjected to mammalian display andthe scFv gene population was recovered and screened, 27/90 clones (30%)were positive in ELISA. This illustrates that it is possible to carryout mammalian display on a library and recover an enriched population ofbinders.

Example 15 describes the introduction of the population selected by 1round of phage display on β-galactosidase into HEK cells using flowelectroporation. The mammalian display cell population was selected inblasticidin as before and was subjected to flow sorting using 10 nM oflabelled β-galactosidase. After 9 days growth 75% of cells were found byflow cytometry to be positive for β-galactosidase binding using 10 nMβ-galactosidase). These were sorted and expanded further. To illustratethe potential to drive stringency, labelling was carried out usingeither 1 nM or 10 nM antigen concentrations. 20.3% and 55.9% of cellsrespectively were sorted from each population. After sorting, mRNA wasprepared immediately from the sorted population without additional cellculture. Following cloning, expression in bacteria and ELISA screening(as before) it was found that the success rate in ELISA increased withincreasing stringency during flow sorting. The clones displaying highestsignal level came from this group and the number of positives clones wasalso improved (FIG. 24). This illustrates the ability to drivestringency of selection within display populations, reflected in thebetter performance of the resulting antibodies.

Example 17. Incorporation of T Cell Receptors (TCRs) Genes for MammalianLibrary Production by Nuclease-Directed Integration

The methods described herein have application beyond display ofantibodies. To demonstrate the potential for screening libraries ofT-cell receptors using nuclease-directed integration, a vector construct(pINT20) allowing expression of T-cell receptors was constructed.

pINT20 (FIG. 25a ) is a dual promoter vector for targeting the humanAAVS locus. It has left and right homology arms as presented in FIG. 3.The left AAVS homology arm is flanked by unique AsiSI and Nsi1 sites andis followed by a splice acceptor and a puromycin gene. The sequencebetween the end of the left homology arm and the splice acceptor is thesame as previously described and the puromycin gene commences with anATG in frame with the upstream exon ((FIG. 25B and as also shown for theblasticidin gene in FIG. 3). Correct nuclease-directed integration willlead to in-frame splicing of a puromycin gene which is spliced to anendogenous upstream exon which itself driven by the endogenous AAVSpromoter allowing selection of clones with correct integration. Thepuromycin resistance gene is followed by an SV40 polyadenylation site.The right AAVS homology arm is flanked by unique BstZ171 and Sbf1 sites.

pINT20 is configured with a pEF promoter (from pSF-pEF, Oxford GeneticsCat. No. OG43), which allows genes of interest to be cloned intoNhe1/Kpn1 sites. The NheI site is preceded by a secretion leadersequence and the Kpn1 site is followed by the polyadenylation signal ofbovine growth hormone (bGH poly A) as shown previously in FIG. 2.Downstream is a CMV promoter (from pSF-CMV-f1-Pac1, Oxford Genetics CatNo OG111) allowing cloning via Nco and Not (as shown in FIG. 2) orHind3. The Nco1 site is preceded by a secretion leader sequence and thecassette is followed by a bGH polyA site.

To exemplify display and enrichment of T cell receptors (TCRs) a TCRrecognising a cancer marker described by Li et al. (2005) and later byZhao et al. (2007)[137, 138] was used. This TCR called c12c2 recognisesthe peptide SLLMWITQV (SE ID NO: 102) presented on HLA-A2 with anaffinity of 450 nM. This peptide represents residues 157-165 fromNY-ESO-1 (NY-ESO-1 157-165). This is an affinity-matured variant of aparental antibody called 1G4 with affinity of 32 μM.

A second TCR was used which recognises another cancer marker. Theparental MEL5 TCR recognises the peptide MART-1 26-35 (“Melanoma antigenrecognised by T cells-1”) presented on HLA-A2 with peptide sequenceELAGIGILTV (SEQ ID NO: 103). This TCR was affinity matured by phagedisplay to give clone α24/β17 with 0.5 nM affinity described in Maduraet al. (2013) [139]. The structure of the complex between TCR andMHC:peptide complex has been solved (pdb code 4JFH) and the clone ishereafter named as “4JFH”. The same parental TCR has also beenengineered based on design by Pierce et al. (2014) [140] and thestructure of the complex solved.

According to Debets and colleagues the attachment of the CD3 ζ domain asshown tends to cause association of the heterologous gene even in thepresence of native human TCRs [141, 142]. The CD3 ζ element used iscomposed of an extracellular domain, a transmembrane domain and acomplete cytoplasmic domain. In addition substitution of human constantdomains by mouse constant domains in the heterologous genes also tendsto drive their association over association with endogenous humanconstant domains [143]. Finally the use of mouse constant domains offersthe option of detecting the heterologous TCR chains against a backgroundof human TCRs. These elements were incorporated into the design of theTCR expression cassette.

Two synthetic genes were designed and synthesised giving rise to geneconstructs with the following structure:

Human TCR Vα-mouse α constant-human CD3 ζHuman TCR Vβ-mouse β constant-human CD3 ζ

The sequence of the synthetic gene incorporating the α chain and the βchain constructs incorporating the variable domains of c12/c2 is shownin FIGS. 25c and d . These are cloned into the Nhe1/Kpn1 sites and theNco1/Hind3 sites of pINT20 respectively. The construct encoding this TCRis called pINT20-c12/c2. In the first instance the synthetic gene wasdesigned to incorporate a Vα Cα domain (flanked by NheI and Not 1 sites)and VP domain (flanked by Nco1/Xho1 sites encoding the TCR c12/c2).These elements can then be replaced by alternative TCRs using theserestriction sites.

Two additional synthetic genes were made encoding the Vα and Vβ domainsof 4JFH (FIGS. 25e and f ). The construct encoding this TCR is calledpINT20-4JFH.

10⁷ HEK293 cells were transfected using 3 μg of pINT20-c12/c2 andpINT20-4JHF as donor DNA (300 ng donor DNA per 10⁶ cells) in the ratiosshown in Table 7. pINT20-c12/c2 is referred to as TCR1 and pINT20-4JHFis referred to as TCR2. 5 μg each of pZT-AAVS1 L1 and pZT-AAVS R1 TALENswere added to 10⁷ cells (500 ng each per 10⁶ cells) with the exceptionof sample 6 where this was replaced by 10 μg of control DNA (pcDNA3.0).DNA was introduced by polyethyleneimine transfection, as describedabove.

TABLE 7 Sample TCR1/TCR2 ratio 1  1:100 2 100:1 3  50:50 4 100% TCR1 5100% TCR2 6  50:50 (no TALE nuclease)

Following 12 days in selection (0.25 μg/ml puromycin) cells werelabelled with target antigen. The peptide:MHC complexes recognised bythe TCRs described above are presented as a phycoerythrin labelledpentamer (ProImmune). c12/c2 recognises peptide SLLMWITQV presented onHLA-A2 representing NY-ESO-1 157-165 (Proimmune product code 390). 4JHF(also known as α24/β17) recognises peptide ELAGIGILTV presented onHLA-A2 representing MelanA/MART 26-35 (Proimmune product code 082). Ineach case the MHC:peptide complex was labelled with phycoerythrin andused according to manufacturer's instructions. FIG. 26 (a-d) shows thateach TCR is specific for the expected MHC: peptide complex (a, d) andfails to bind to the non-cognate peptide in complex (FIG. 26 b, c). DNAencoding each TCR was mixed with a 100-fold excess of DNA encoding theother (samples 1-2, Table 7). HEK293 cells were transfected and selectedin puromycin. Sorting of antigen positive cells was performed after 14days of puromycin selection (FIG. 26 g, h).

In order to quantitate the level of enrichment within the flow-sortedoutput populations, the TCR genes were recovered by PCR amplificationand the relative amounts of each TCR species determined followingcloning. Total RNA was isolated from the sorted population. cDNAsynthesis was performed as described in example 16. Primers to amplifythe TCR alpha and beta chain were 1999/2782 and 41679/2789 respectively(Table 8). PCR amplification employed KOD hot start polymerase using themanufacturer's recommended protocol (EMD Millipore, 71086, EMDMillipore). PCR reaction conditions were 95° C. (2 mins) and 25 cyclesof 95° C. (20s), 60° C. (10s), 70° C. (15s) followed by 70° C. (5 mins).The amplified TCR alpha and beta chains were digested with NheI/Not1 orNco1/XhoI and sub-cloned into vectors with compatible sites (in thiscase pBIOCAM1-Tr-N NheI/NotI or pBIOCAM2-Tr-N(NcoI/XhoI) cut vectorsrespectively). PCR of individual clones were amplified with a c2c12(TCR1) specific TCR alpha primer (2781) or a 4JFH (TCR2) specific TCRalpha primers (4JFH-Vα-F) and vector specific primers to assay for TCRclone identity. After sorting of the sample 1 (Table 7) where a ratio of1:100 TCR1/TCR2 donor plasmid was employed (Table 7) with enrichment forTCR1 specific clones using peptide SLLMWITQV presented on HLA-A2(Proimmune, product code 390), the proportion of TCR1 clones, asdetermined by colony PCR, increased to 11/15 (73%). Enrichment bysorting the sample 2 where a ratio of 100:1 TCR1/TCR2 donor plasmid wasemployed (Table 7), with peptide ELAGIGILTV presented on HLA-A2(Proimmune product code 082) resulted in an increase of the proportionof TCR2 clones to 4/15 (27%), determined by colony PCR.

To demonstrate library selection using nuclease-directed integration, amutant library based on c12/c2 was created by cloning a repertoire ofgenes encoding mutant TCR alpha chains along with a repertoire of genesencoding mutant TCR beta chains. Such a library could be created usingoligonucleotide-directed mutagenesis approaches. e.g., methods based onKunkel mutagenesis [144]. As an alternative and by way of example, a PCRassembly approach was used to create a mutant TCR alpha chain (as aNhe1/Kpn1 fragment) and a mutant TCR beta chain (as a Kpn/Hind 3fragment, including the CMV promoter) which is cloned into the NheI/Hind3 site of pINT20. This was done using primers shown in Table 8.

TABLE 8  Primers used in library construction and  clone recovery4JFH-Vα-F ACACACGCTAGCCAGAAAGAGGTGGAACAG (SEQ ID NO: 104)  1999AAAAAGCAGGCTACCATGAGGGCCTGGATCTTC (SEQ ID NO: 87) TTTCTCC 2770CAAAGAACAGCTCGCCGGTSNNCCCGASSNNGG (SEQ ID NO: 105)  AGCTGGCGCAAAAGTAC2771 CTCGCCCGAAGGTGGGAATGTANGWTCCSNNSN (SEQ ID NO: 106) NAAGTGGGCGCACGGCGCAC 2780 CTGGCAGCTAGCAAGCAGGAAG (SEQ ID NO: 107)  2781TACATTCCCACCTTCGGGCGAG (SEQ ID NO: 108)  2782 TTTTTTGCGGCCGCGGACAGGTTCTG(SEQ ID NO: 109)  2783 CGTAAGCTGGTACCTTATTATCTAGGG (SEQ ID NO: 110) 2785 CCCTAGATAATAAGGTACCAGCTTACG (SEQ ID NO: 111)  2787ACCGGCGAGCTGTTCTTTG (SEQ ID NO: 112)  2788 AGTGACAAGCTTTTATTATCTGGGTG(SEQ ID NO: 113)  2789 CAGGTCCTCGAGCACTGTC (SEQ ID NO: 114) 41679ATGAGTTGGAGCTGTATCATCC (SEQ ID NO: 100) (N = A, C, G, T. S = C OR G, W= A OR T)

A mutant oligonucleotide was designed (primer 2771) which randomised 2amino acid positions within CDR3 of the c12/c2 alpha chain and providethe option of either serine or threonine at another position (primer2771 is also represented by the lower strand of FIG. 25 g). Primer 2771was used in conjunction with primer 2780 to create mutant TCR alpharepertoire going from the NheI cloning site incorporating the region ofCDR3 mutagenesis with an invariant sequence at the end. Primer 2781 iscomplementary to the invariant 5′ end of primer 2771. PCR with primers2781 and 2783 provided the remainder of the TCR alpha-CD3 zeta cassetteup to the Kpn1 site. PCR assembly of the 2 PCR fragments is used tocreate the TCR alpha-CD3 zeta fragment which can be cloned into pINT20following digestion with Nhe1 and Kpn1.

A second mutant oligonucleotide (primer 2770) was designed whichrandomised 2 amino acid positions within CDR3 of the c12/c2 beta chainand provide the option of either valine or leucine at another position(primer 2770 represented by the lower strand of FIG. 25h ). Primer 2770was used in conjunction with primer 2785 to create a mutant TCR betarepertoire from the Kpn1 cloning site incorporating the region of CDR3mutagenesis with an invariant sequence at the end. Primer 2787 iscomplementary to the invariant 5′ end of primer 2700. PCR with primers2787 and 2788 provided the remainder of the TCR beta-CD3 zeta cassetteup to the Hind 3 site. PCR assembly of these 2 PCR fragments is used tocreate a TCR beta-CD3 zeta fragment which can be cloned into pINT20. Acomplete repertoire incorporating mutations at both CDR 3 of alpha andbeta chains was created by cloning of the NheI/Kpn fragment, theKpn1/Hind3 fragment into NheI/Hind3 digested pINT20. Following ligationthe library was cloned into electrocompetent DH10B cells. Plasmid DNAwas prepared and the DNA transfected into HEK293 cells along withvectors encoding TALE nuclease as described earlier (an equimolar mix ofpZT-AAVS1 L1 and pZT-AAVS R1 Systems Bioscience Cat. No. GE601A-1).

Following ligation of the mutant alpha and beta chains of the c12/c2mutant library into pINT20, DNA was electroporated into DH10B cell,plasmid DNA was prepared and the library was co-transfected with TALEnuclease targeting the AAVS locus. Transfection was performed usingMaxcyte electroporation. Growth and selection were as described above.Quantitation of the library size, by titration of transfected cells andplate-based selection in puromycin, indicated that a library size 5×10⁵was created. After puromycin selection for 11 days cells were labelledwith an APC-labelled antibody specific to the β chain of the mouse TCR(Life Technologies Cat H57-957). FIG. 26 i-j shows 38% of clones in thepopulation express a T cell receptor. Of this TCR positive population,13% also bound peptide1 (5% of the total population). By this approachclones with improved expression or peptide:MHC binding activity can beisolated.

From FIG. 26 it can be seen that each T cell receptor recognised onlyits cognate antigen. Furthermore when a mixture of the two differentspecificities are used it is possible to distinguish each of themthrough the labelled antigen. This approach also allows TCR clones withimproved affinity (or expression) to be distinguished within the mutantTCR library by identifying a subset of the library that was labelled toa greater extent than the parental clone.

T cell receptor genes were also introduced into Jurkat cells byelectroporation. Jurkat cells were centrifuged and re-suspended in afinal volume of 10⁸ cells/ml in the manufacturer's electroporationbuffer (Maxcyte Electroporation buffer, Thermo Fisher Scientific Cat. noNC0856428)). An aliquot of 4×10⁷ cells (0.4 ml) was added to the 0C400electroporation cuvette with 40 μg DNA (i.e., 1 μg/10⁶ cells). DNAconsisted of a mixture of donor plasmid DNA (pINT20-c12/c2 orpINT20-4JHF or pINT20-c12/c2 TCR library, 9.2 μg) and an equimolar mixof DNA (30.8 μg total) of DNA encoding the AAVS-SBI TALENs (pZT-AAVS1 L1and pZT-AAVS R1 Systems Bioscience Cat No GE601A-1). In samples withoutadded TALENs the input DNA was brought to 1 μg/10⁶ cells using controlplasmid pcDNA3.0. An alternative method of introducing T cell receptorgenes into Jurkat cells used the 4D-Nucleofector (Lonza). Here, thetransfection protocol followed the manufacturer's instructions accordingto the SE cell-line kit (Lonza, Cat. PBC1-02250). Briefly, 2 μg of DNA,consisting of a mixture of donor plasmid DNA (pINT20-c12/c2 orpINT20-4JHF or pINT20-c12/c2 TCR library, 0.46 μg) and an equimolar mixof DNA (1.54 μg total) of DNA encoding the AAVS-SBI TALENs (pZT-AAVS1 L1and pZT-AAVS R1 Systems Bioscience Cat No GE601A-1) was transfected per10⁶ Jurkat cells. The pulse code setting was CL120 and cell typeprogramme was specific for Jurkat E6.1 (ATCC) cells.

FIG. 26 demonstrates that TCR expression was achieved and recognition ofthe appropriate peptide:MHC molecule was achieved. This was dependent onthe use of TALE nuclease (compare FIG. 26 m and n). Signalling throughthe introduced TCR was also achieved using the relevant peptide:MHCmolecule.

pINT20-c12/c2 transfected Jurkat cells or untransfected Jurkat cellswere plated in a 96-well plate at a density of 1×10⁶/ml, 200 μl perwell. Cells were stimulated with either 2 μl or 6 μl per well of PElabelled peptide 1-MHC pentamer (ProImmune) or 2 μg/ml anti-human CD3(BD Pharmingen, Cat 555329) in the presence and absence of anti-humanCD28 (BD Pharmingen Cat 555725) at 2 μg/ml. After a 24 hour incubationat 37° C. and 5% CO₂, the activation of Jurkat cells was detected byinvestigating CD69 expression. Cells were stained with 50 μl PBS+1%BSA+0.5 μl of anti-human CD69-APC (Invitrogen, Cat. MHCD6905) per wellfor 45 minutes at 4° C.FIG. 26 (sample o and p) demonstrates up-regulation of CD69 uponstimulation with CD3. The figure also shows the effect of adding 2 μl(q) or 6 μl (r) of peptide1:MHC. A population of double-positive cellswhich bind peptide:MHC and express CD69 is obvious. This example showscells incubated in the presence of CD28 but the same effect was observedin the absence of CD28 (not shown).

This example demonstrates the potential of nuclease-directed integrationof libraries of an alternative type of binder, i.e., T cell receptors.We demonstrate that it is possible to express and detect TCR expressionon the cell surface using specific antibodies. We also demonstrate thatthese T cell receptors specifically recognise their respective targets.We have also constructed a mutant library allowing selection of improvedbinders. Finally we have demonstrated that library screening based onactivation of TCR signalling in T cells is possible. Here we have used acultured human T cell line. It is also possible to introduce DNA intoprimary T cells by Maxcyte electroporation. Methods for the isolationand preparation of primary T lymphocytes are known to those skilled inthe art (e.g., Cribbs et al., 2013, Oelke et al. 2003 [145, 146].Exposure of TCR transfected lymphocytes to multimeric peptide:MHC canthen be used to achieve activation either through exposure topeptide:MHC multimers [146] or to antigen presenting cells loaded withthe appropriate peptide [146, 147]. Activation can be detected eitherthrough expression of reporter genes or through up-regulation ofendogenous genes such as CD69 [104, 148].

Example 18. Display of Libraries of Chimeric Antigen Receptors onMammalian Cells

Activation of T cells normally occurs through interaction of the T CellReceptor (TCR) with specific peptide:MHC complexes. This in turn leadsto signalling directed via CD3 and other T cell signalling molecules. Asan alternative to target recognition directed by the TCR it has beenshown that alternative binding molecules such as single chain Fvs can bepresented on T cells as fusions to downstream signalling molecules in away that re-directs T cell activation to the molecule recognised by thescFv (or alternative binder). In this way T cell activation is no longerlimited to the molecular recognition directed towards peptide:MHCcomplexes by TCRs but can be directed to other cell surface molecules.This alternative format wherein a non-TCR binding entity is fused to asignalling component is referred to as a “chimeric antigen receptor”(CAR). In the case of T cells this has been shown to be an important andvaluable means of re-directing T cell activation.

For any given target it is still not clear what the optimal epitope oraffinity features should be for incorporation into a CAR [103]. Featuresof the CAR design such as linker length, or choice of transmembranedomain may in turn affect what constitutes an optimal epitope. Thecombination of antigen density on target and non-target cells togetherwith the choice of signalling domain could affect the optimal affinityrequirements. The ability to present libraries of chimeric antigenreceptors on T cells affords an opportunity to identify optimal bindingspecificity, binder format, linker length/sequence, variants of fusedsignalling module, etc., either alone or in combination. Here wedemonstrate the utility of nuclease-directed integration forconstruction of libraries of chimeric antigen receptors in mammaliancells. The vector pINT21 (FIG. 27a ) is a single CMV promoter vector forconvenient expression and secretion of binders such as scFvs flanked byNco1/Not1 restriction sites to allow in frame expression with anupstream leader sequence and a downstream fusion partner (as shownearlier in FIG. 8). The CAR expression cassette in pINT21 is flanked byAAVS homology arms as described earlier in FIG. 3.

The vector pINT21-CAR1 (FIG. 27a, c ) fuses binders such as single chainFvs to the transmembrane domain and intracellular domain of CD3 (FIG.27c and as described for TCR expression in FIG. 25). This format isoften referred to as a “first generation” chimeric antigen receptor.Signalling domains from other co-stimulatory molecules have also beenused to provide additional signals and these have been shown to giveimproved signalling. These have been referred to as second and thirdgeneration chimeric antigen receptors. For example pINT21-CAR2 (FIG. 27b, d) fuses the binder (conveniently cloned in this case as an Nco1/Not1 fragment to a previously described second generation domain (WO2012/079000 A1) consisting of:

The hinge and transmembrane domain from CD84-1BB signalling domainCD3ζ signalling domain

By way of example a number of different binder groups were cloned intothe Nco/Not1 sites of pINT20-CAR1 and pINT20-CAR2. CD19 has previouslybeen used in a number of different studies to target B-cell malignanciesreferences in Sadelain et al. (2013) [103]. A previously describedanti-CD19 antibody (WO 2012/079000 A1) (called FMC63) was prepared as asynthetic scFv gene in either a VH-linker-VL configuration or aVL-linker-VH configuration (FMC63 H-L or FMC L-H respectively, FIG. 27Eshows the sequence of FMC63 H-L. FMC63 L-H was configured with thevariable domains in VL-linker-VH configuration flanked by Nco1 and Not1at 5′ and 3′ ends respectively.

As controls, scFvs with alternative binding specificities were alsocloned into pINT20. These include anti-FGFR1_A [105] and an anti-desmincontrol antibody [7]. In addition, Adhirons [152] recognising lox1 (FIG.29 a, b) were introduced as an example of an alternative format ofbinder configured as a CAR fusion (see example 19 for description).

To demonstrate the creation of libraries of binders presented in a CARformat, populations of scFv-formatted antibodies selected on mesothelinand CD229 were cloned. Mesothelin is a cell surface glycoprotein whichis highly expressed in a number of cancers including mesothelioma. Anumber of antibody-based formats are under development and in clinicaltrial including CARs directed to mesothelin [149]. CD229 representsanother potential tumour-associated antigen which could be targeted byimmune therapy in cancers such as chronic lymphocytic leukaemia andmultiple myeloma [150, 151].

A population of antibodies recognising either mesothelin or CD229 wascreated by selection using the “McCafferty” phage display library asdescribed in example 6 and ref 7). Two rounds of selection were carriedout and the scFv-encoding genes recovered using primers M13leadseq andNotmycseq (example 6). Products were cut with Nco1/Not 1, gel purifiedand cloned into pINT21-CAR2. These were directed into the AAVS locus ofHEK293 cells by TALE nuclease cleavage to generate a library of 4.8×10⁵for CD229 and 6.4×10⁵ for mesothelin (representing a 30× and 53×increase in library compared to samples transfected in the absence ofTALE nuclease).

pINT20-CAR1 and pINT20-CAR2 were introduced into HEK293 cells by PEItransfection. Here donor plasmid DNA (pINT20-CAR1 or pINT20-CAR2, 6 μgwas mixed with an equimolar mix of DNA (20 μg total) of DNA encoding theAAVS-SBI TALENs (pZT-AAVS1 L1 and pZT-AAVS R1 Systems Bioscience Cat NoGE601A-1) in Freestyle 293 media (Lifetech, Cat. 12338-026), linear PEI(52 μl, 1 mg/ml, Polysciences Inc.) added and incubated at roomtemperature for 10 mins. The mixture is then added to 20 ml HEK293suspension cells (1×10⁶ cell/ml) in a 125 ml vented Erlenmeyer flask.pINT20-CAR1 and pINT20-CAR2 were also introduced into Jurkat cells byelectroporation. Jurkat cells were centrifuged and re-suspended in afinal volume of 10⁸ cells/ml in the manufacturer's electroporationbuffer (Maxcyte Electroporation buffer, Thermo Fisher Scientific Cat. noNC0856428)). An aliquot of 4×10⁷ cells (0.4 ml) was added to the 0C400electroporation cuvette with 40 μg DNA (i.e., 1 μg/10⁶ cells). DNAconsisted of a mixture of donor plasmid DNA (pINT20-CAR1 andpINT20-CAR2, 9.2 μg) and an equimolar mix of DNA (30.8 μg total) of DNAencoding the AAVS-SBI TALENs (pZT-AAVS1 L1 and pZT-AAVS R1 SystemsBioscience Cat No GE601A-1). In samples without added TALENs, the inputDNA was brought to 1 μg/10⁶ cells using control plasmid pcDNA3.0.

Fluorescent labelling of the various antigens was performed usingLightning-Link Rapid Dye-Light 633 conjugation kit (Innova Biosciences,cat. 325-0010). Preparation of FGFR1 and FGFR2 is described in example15. Lox1 and CD229 were from R and D Systems (Cat. Nos. 1798-LX-050, and898-CD050 respectively), CD19-Fc and mesothelin were from(AcroBiosystems Cat. No. CD9-H5259 and. MSN-H526× respectively).

FIG. 28 b illustrates display of nuclease-directed anti-FGFR1 antibody[105] within a second generation CAR construct (pINT21-CAR2-FGFR1_A).FIG. 28d also illustrates display of an alternative scaffold molecule(an Adhiron ref [152]) as a fusion with a second generation chimericantigen receptor (pINT21-CAR2-lox1). FIG. 28 f and g also illustratepositive clones within a library of scFvs selected on mesothelin orCD229.

In this example CARs were introduced into HEK cells and Jurkat cells butthis could equally be done by introducing the constructs into primarycells such as human T lymphocytes (e.g., as described by Sadelain et al.(2013) [103], for example using electroporation [135]. Expressionconstructs for CAR expression in lymphocytes may be further optimised,e.g. by optimising mRNA stability and translation through variation in5′ and 3′untranslated regions, poly A length etc., as has previouslybeen described [135]. Signalling of CAR constructs introduced intoprimary T lymphocytes or T lymphocyte cell lines can be induced byexposure to cells expressing target antigen or using multimeric antigen,e.g. antigen immobilised on a surface or presented on beads [104, 148].

Example 19. Display of Libraries of Alternative Scaffolds Constructed inMammalian Cells Via Nuclease-Directed Integration

The method described for constructing libraries of binders can beemployed beyond display of antibodies and T cell receptors. A number ofalternative scaffolds have been described allowing construction oflibraries of variants from which novel binding specificities have beenisolated, e.g., Tiede et al. (2014) [152] and references therein. In theexample described by Tiede et al. (2014) a stable, versatile scaffoldbased on a consensus sequence from plant-derived phytocystatins wasused. This scaffold is referred to as an Adhiron and FIG. 29a shows asynthetic gene encoding an Adhiron which was selected to bind to lox1(WO 2014125290 A1). FIG. 29 B shows an alternative lox1 binder (lox1B).Both were synthesised and cloned into the Nco1/Not1 site of pINT20_CAR2to create a fusion with the downstream partner.

A library can be constructed by randomising loop residues (e.g., byKunkel mutagenesis or PCR assembly as described above in example 17). Byway of example, FIG. 29c shows the design of a mutant oligonucleotidesuseful to create a library following the same approach as described inexample 17. In this case, randomisation is achieved by introducing avariable number of NNS residues, although alternative strategies knownto those skilled in the art could be used.

As another example FIG. 29 d and e demonstrates the means to create alibrary of binders by nuclease-directed integration based on a knottinscaffold [156]. Knottins are peptides of approximately 30 amino acidswhich are stabilised by three disulphide bonds, with one threadedthrough the other two to create a “knotted” structure. FIG. 29 d showsthe trypsin binding knottin MCoTI-II with an Nco1 site at the 5′ end anda Not site at the 3′ end allowing in-frame expression with the vectorsdescribed herein. As an example for library construction the 6 aminoacids of the first loop (underlined in FIG. 29d ) can be mutated withvariable number of amino acids. FIG. 29e illustrates a mutagenicstrategy replacing the 6 amino acids of loop 1 with 10 randomised aminoacids using the codons VNS (where V=A, C or G and S═C or G). The VNScodon encompasses 24 codons encoding 17 amino acids which excludecysteines. This strategy is for illustrative purposes and alternativemutagenic strategies will be known to those skilled in the art.

Example 20. Nuclease-Directed Introduction of Antibody Libraries UsingCRISPR/Cas9

Nuclease-directed integration via CRISPR/Cas9 was demonstrated using the“Geneart CRISPR nuclease vector kit” (Lifetech A21175). In this system,a U6 RNA polymerase III promoter drives expression of a targetcomplementary CRIPSR RNA (crRNA) which is linked to a trans-activatingcrRNA (tracrRNA). The crRNA and tracrRNA together make up a guide RNAwhich directs the cleavage specificity of a Cas9 protein encoded on thesame “GeneArt CRISPR nuclease vector” (see manufacturer's instructions).The vector is provided as a linearised plasmid into which a shortdouble-stranded oligonucleotide with appropriate 3′ overhangs is cloned.Cleavage specificity is then determined by the sequence of the clonedsegment. Two different targeting sequences were designed to directcleavage to the human AAVS locus describe above.

The sequences were:

CRISPR1 double-stranded DNA insert: (SEQ ID NO: 115)5' GGGGCCACTAGGGACAGGATGTTTT (SEQ ID NO: 116)3' GTGGCCCCCGGTGATCCCTGTCCTAC CRISPR2 double-stranded DNA insert:(SEQ ID NO: 117) 5' GTCACCAATCCTGTCCCTAGGTTTT (SEQ ID NO: 118)3' GTGGCCAGTGGTTAGGACAGGGATC

The resulting guide RNAs target cleavage within the same region of theAAVS locus as the TALE nucleases described above (with CRISPR2 being inthe reverse orientation from CRISPR1). Thus the AAVS homology arms usedpreviously to direct integration of the expression cassette can be usedfor integration directed by these CRISPR guide RNAs. Linearised vectorand double stranded oligonucleotides were ligated and transformed intoelectrocompetent DH10B cells. Cloning of the correct insert wasconfirmed by sequencing and plasmid DNA was prepared. The Cas9/CRISPR2construct (encompassing the CRISPR2 oligonucleotide) was transfectedtogether with donor DNA encoding a β-galactosidase library selected by 1round of phage display selection (example 15). These were transfectedinto HEK293 cells using Maxcyte electroporation system with the OC-400assembly. 4×10⁷ cells were transfected with 23.2 μg of donor DNArepresenting a population of scFv genes selected by one round of phagedisplay on β-galactosidase cloned into pD6. Cells were co-transfectedwith either 77 μg of Cas9-CRISPR2 plasmid, or 77 μg TALEN plasmid (38.5μg each of pZT-AAVS1 L1 and pZT-AAVS) or 77 μg control plasmid

TABLE 9 Transfection Number of clones Sample no Nuclease used efficiency% per 10¹⁰ cells 1 Cas9/CRISPR 2 10.53 (29×)   10 × 10⁸ 2 AAVS TALEN 5.1 (14×)  5.1 × 10⁸ 3 None (pCDNA3.0) 0.36 0.36 × 10⁸

Titration of the number of transformants formed by Cas9/CRISPR2transfection (by measuring blasticidin resistance colonies) revealedthat 1053 blasticidin resistant colonies were generated from plating10,000 cells, equating to a transfection efficiency of 10.5% (Table 9).In the case of TALE nuclease-directed a transfection efficiency of 5.1%was achieved. In contrast, in the absence of the Cas9/CRISPR2 constructonly 0.36% transfection efficiency was achieved.

As an alternative to transfection using plasmid DNA to introduce theCas9 protein and guide RNA into cells it is also possible to directlyintroduce a nucleoprotein complex consisting of Cas9 protein (Toolgen,Inc.) and a guide RNA. Guide RNA was prepared by Toolgen, Inc. using invitro transcription from a T7 promoter as a single transcript whichincluded the TRACR sequence (italicized) preceded by sequencecomplimentary to the target DNA (in bold) as shown below.

CRISPR 1 RNA (SEQ ID NO: 119: 5' GGGGGGCCACUAGGGACAGGAUGUUUUAGAGCUAGAAAUAGCAA GUUAAAAUAAGGCUAGUCCGUUAUCAACUUGAAAAAGUGGCACCGAGUCGGUGCUUUU CRISPR 2 RNA (SEQ ID NO: 120): 5' GGGUCACCAAUCCUGUCCCUAGGUUUUAGAGCUAGAAAUAGCAA GUUAAAAUAAGGCUAGUCCGUUAUCAACUUGAAAAAGUGGCACCGAGUCGGUGCUUUU

6.6 μg of Cs9 protein, 4.6 μg of RNA and 10 μg of donor DNA (encodingthe anti-FGFR1_A antibody in pD6) were introduced into 10⁷ HEK293 cellsby Maxcyte electroporation as described above. Transfection efficienciesof 2.2% and 2.9% were achieved for CRISPR 1 and CRISPR2 RNA respectivelywith 0.7% and 0.8% in the absence of added Cas9:RNA protein complex.

These guide RNAs target the same sequences encoded by CRISPR1 andCRISPR2. As an alternative, crRNA and tracrRNA can be made by chemicalsynthesis (e.g., GE Dharmacon).

Example 21: Nuclease Mediated Antibody Gene Insertion by Ligation orMicrohomology-Mediated End-Joining (MMEJ)

Although homologous recombination (HR) is useful for the preciseinsertion of large DNA fragments, this requires the construction oflarge targeting vectors incorporating long homology arms. This can makethe construction of large libraries more difficult due to the reducedtransformation efficiency of larger DNA constructs. Alternatively,simple ligation reactions can occur between the chromosomal DNA andtargeting vector, if a nuclease recognition sequence is incorporatedinto the targeting vector. The ligation reactions can either be“sticky-end” employing, for example, zinc finger nucleases (Orlando etal., 2010) [45] or TALENs (Cristea et al., 2013) [22] which can makedouble-strand breaks (DSBs) that leave 5′ overhangs or “blunt-end”employing CRISP/Cas9 ribonucleoprotein. An example of nuclease geneintegration by ligation using I-SceI meganuclease was shown by theconstruction of vector pD7-Sce1. pD7 is derived from pD6 (FIG. 8) butthe left and right AAVS homology arms were replaced with short doublestranded oligonucleotides. The left AAVS homology arm of the pD vectorseries is flanked by EcoR1 and Nsi1 restriction enzymes (see FIG. 3). Toconvert pD6 to pD7-Sce1, this was replaced by a double-strandedoligonucleotide insert formed by primers 2778 and 2779 encoding anI-SceI meganuclease recognition sequence with “sticky ends” compatiblewith the “sticky ends” formed by EcoRI/NsiI. The right hand AAVShomology arm is flanked by Asc1 and Mlu1 restriction sites (FIG. 3). Theright homology arm was replaced by a double-stranded oligonucleotideinsert with “sticky ends” compatible with the “sticky ends” formed byAscI/MluI digestion and is formed by primers 2723 and 2724.

TABLE 10  Primers for pD7 and pINT19 construction 2723CGCGCCAGAAGTCTCACCAAGCCCA (SEQ ID NO: 121) 2724CGCGTGGGCTTGGTGAGACTTCTGG (SEQ ID NO: 122) 2768 AATTCTCCCCTCCACCCCACAGT(SEQ ID NO: 123) AGGGACAGTGGGGCC AGGATTGGTG ACAGAAAATGCA 2769TTTTCTGTCACCAATCCTGGCCCCACTGT (SEQ ID NO: 124) CCCTACTGTGGGGTGGAGGGGAG2778 AATTCTAGGGATAACAGGGTAATATGCA (SEQ ID NO: 125) 2779TATTACCCTGTTATCCCTAG (SEQ ID NO: 126) 2808 AATTCTTTTCTGTCACCAATCCTGGGG(SEQ ID NO: 127) CCACTAGGGACACTGTGGGGTGGAGGG GATGCA 2809TCCCCTCCACCCCACAGTGTCCCTAGTGGC (SEQ ID NO: 128)CCCAGGATTGGTGACAGAAAAGAATTG

Antibodies recognising Fgfrl and Fgfr2 (example 15) were cloned into pD7to create pD7-SceI anti-Fgfr1 and pD7-SceI anti-Fgfr2 respectively.These were co-transfected with the I-Sce1 expression plasmid (example11, FIG. 16) into the HEK293 clone 6F cell line (see example 11) whichcontains an integrated I-Sce1 recognition site.

Ligation of DSBs in the chromosome and targeting vector generated byzinc finger nuclease or TALE nucleases can also be achieved. Byinverting the zinc finger nuclease or TALEN recognition sites on thetargeting vector this can ensure that the product of the insertion is nolonger a target for cleavage in a method termed “Obligate ligation-gatedrecombination” or ObLiGaRe (Maresca et al., 2013) [153]. pD7-ObLiGaRevectors can be generated in the same way as described above for thecreation of pD7-Sce1. In this case, the left hand homology arm isreplaced by an oligonucleotide consisting of primers 2808 and 2809encoding an inverted TALEN recognition site (shown in bold) and spacerregion. The right hand homology arm is replaced with primers 2723 and2809 as described above.

An alternative to simple ligation reactions between DSBs in thechromosome and targeting vector, mediated by non-homologous end joining(NHEJ) is microhomology-mediated end-joining (MMEJ). MMEJ is a DSBrepair mechanism that uses microhomologous sequences between 5 to 25 bpfor error-prone end joining (McVey and Lee, 2008) [154]. A strategy forprecise gene integration has been devised where the genomic sequence andthe targeting vector contain the same TALE nuclease pair recognitionsequence, but a different vector spacer sequence in which the anteriorand posterior half are switched. The genomic sequence and vector can becut by the same TALEN pair and MMEJ takes place via the microhomologousDNA ends. The resultant integrated targeting vector is no longer atarget for TALE nuclease because of the shortened spacer region whichare not optimal for TALE nuclease cleavage (Nakade et al., 2014) [155].

The MMEJ AAVS targeting vector pD7-MMEJ can be generated in the same wayas described above for the creation of pD7-Sce1. In this case the lefthand homology arm is replaced by an oligonucleotide consisting ofprimers 2768 and 2769 encoding the TALEN recognition site (shown inbold) and switched spacer region (underlined). The right hand homologyarm is replaced with primers 2723 and 2809 as described above.

Example 22: Design of Primers for Creation of Single (CMV) PromoterCassette Flanked by ROSA26 Arms (pINT19-ROSA)

This example is intended to demonstrate that antibody or alternativebinding molecule genes can be integrated into the genome of mammaliancells by nuclease directed methods and the resultant clones screened fora desired function by either reporter or phenotypic screening. Anexample of this was previously demonstrated where antibody genes wereintegrated into the chromosome of mouse embryonic stem (ES) cells andindividual ES colonies screened for their ability to maintainpluripotency when subjected to differentiation conditions [105].Antibody genes recovered from ES colonies which maintained a pluripotentphenotype were shown to block the FGFR1/FGF4 signaling pathway. Aproblem with this previously reported method is that homologousrecombination can result in small library sizes, thus limiting itsability to directly screen for rare clones present in large bindingmolecule libraries. Nuclease-mediated gene integration methods forantibody and binding molecule gene integration are more efficientresulting in larger library size generation and thus more likely togenerate mammalian cell libraries capable of identifying functionalantibodies by phenotypic or reporter cell screening.

The donor targeting vector pINT19 is designed to integrate antibodygenes into the mouse ROSA26 locus by nuclease directed methods fordirect functional screening. pINT19 is a single CMV promoter vector forscFv-Fc fusion expression. The expression cassette is flanked by mouseROSA26 arms. Since the upstream exon is untranslated, the puromycin geneis preceded by a splice acceptor and further down has a KOZAK sequenceleading into the puromycin gene.

The AAVS left homology arm and puromycin resistance gene of pINT18 wasreplaced by a cassette encoding the ROSA26 left homology, spliceacceptor, optimized kozak consensus sequence and puromycin resistancegene. The ROSA26 left homology arm was initially amplified from pGATOR(Melidoni et al., 2013 (105)) as two fragments which knocked out aninternal NotI site. The two fragments, generated by primers J60/2716 and2715/2706 were combined in an assembly PCR with primers J60 and 2706 anddigested with AsiSI and NsiI. The splice acceptor was amplified frompGATOR using primers 2709 and 2710 and the puromycin resistance cassetteamplified with primers 2745 (which included a region homologous to thesplice acceptor and optimized Kozak consensus) and J59. The spliceacceptor region and puromycin resistance cassette were combined inassembly PCR using primers 2709 and J59 and digested with Nsi1 and Bgl2.The ROSA26 left arm homology and splice acceptor-puromycin cassetteswere ligated with pINT18 (AsiS1/Bgl2) vector.

To complete the ROSA targeting vector the right hand ROSA26 homologyarm, downstream of CMV-scFv-Fc cassette, was introduced to replace thepINT18 AAVS right homology arm. This was performed by PCR of the ROSAright homology arm, present in pGATOR (Melidoni et al., 2013) usingprimers J61 and J62 to amplify a fragment with BstZ171 at one end andSbf1 at the other. Primer 61 was positioned to exclude an endogenousSbf1 site 65 bp up from ROSA ZFN cleavage site. FIG. 31 shows thesequences of ROSA26 left and right homology arms.

TABLE 11  Primers for nuclease-directed targeting of the mouse ROSA 26 locus J60 ACACACGGTACCGCGATCGCGCT AsiSI-rosa26-(SEQ ID   GATTGGCTTCTTTTCCTC L-F NO: 129) 2706 TTTTTTATGCATTCTAGAAAGACNsiI-rosa26- (SEQ ID  TGGAGTTGCAGA L-R NO: 130) 2715 GAGCGTCCGCCCACCCTCROSA-Left- (SEQ ID  NotI_knockout_F NO: 131) 2716 GAGGGTGGGCGGACGCTCROSA-Left- (SEQ ID  NotI_knockout_R NO: 132) 2709 TTTTTTATGCATTAAGGGATCTSplice_acceptor- (SEQ ID  GTAGGGCGCAG F-NsiI NO: 133) 2710GTGAATTCCTAGAGCGGCCTC Splice_acceptor- (SEQ ID  R NO: 134) 2745GAGGCCGCTCTAGGAATTCACG Overlap-Puro- (SEQ ID  CCGCCACCATGACCGAGTACAAF + kozak NO: 135) GCCCAC J59 AAAAAAAGATCTGTGTGTTTCGAA Bgl2-Puro-R(SEQ ID  TCAGGCACCGGGCTTGCGGGTCAT NO: 136) J61 ttttttGTATACGGGAATTGAACAROSA-Right_F- (SEQ ID  GGTGTAAAATTG BstZ171 NO: 137) J62TTTTTTCCTGCAGGAGGTTGGATT ROSA-Right_R- (SEQ ID  CTCAATACATCTATTGTTG SbfINO: 138) 2701 GCCGACGTCTCGTCGCTGATGTTTT (SEQ ID  NO: 139) 2702ATCAGCGACGAGACGTCGGCCGGTG (SEQ ID  NO: 140) 2703CGCCCATCTTCTAGAAAGACGTTTT (SEQ ID  NO: 141) 2704GTCTTTCTAGAAGATGGGCGCGGTG (SEQ ID  NO: 142)

Integration of pINT19, encoding antibody or alternative bindingmolecules, into the mouse ROSA26 locus could be achieved bynuclease-directed introduction of antibody libraries using CRISPR/Cas9as described in Example 20. Here, nuclease-directed integration viaCRISPR/Cas9 could be demonstrated using the “Geneart CRISPR nucleasevector kit” (Lifetech A21175). In this system, a U6 RNA polymerase IIIpromoter drives expression of a target complementary CRIPSR RNA (crRNA)which is linked to a trans-activating crRNA (tracrRNA). The crRNA andtracrRNA together make up a guide RNA which directs the cleavagespecificity of a Cas9 protein encoded on the same “GeneArt CRISPRnuclease vector” (see manufacturer's instructions). The vector isprovided as a linearised plasmid into which a short double-strandedoligonucleotide with appropriate 3′ overhangs is cloned. Cleavagespecificity is then determined by the sequence of the cloned segment. 2different targeting sequences were designed to direct cleavage to themouse ROSA26 encoded by primers 2701/2702 and 2703/2704 (see Table 10).

As an alternative, Zinc finger nucleases which cleave within the ROSA26locus have been described [34]. These could be constructed in anappropriate expression vector as described for Sce-I meganuclease (FIG.16, example 11).

Nuclease-mediated integration of donor plasmid pINT19 would give rise toclones expressing secreted antibody which could bind endogenous receptoror ligand resulting in either antagonism [105,107] or agonism[47,106,108] of receptor signalling pathways. To enable the linkagebetween cellular phenotype and the functional activity of the secretedantibody, cells can be plated at a low density in semi-solid media sothat individual colonies propagate and antibody expression can beinitiated via an inducible promoter [105]. Alternatively, a constitutivepromoter could be employed for antibody gene expression. The semi-solidmedia would maintain an elevated local concentration of theendogenously-expressed antibody, so that any phenotypic change specificto a colony arising from an individual cell would be caused by theunique antibody expressed from that particular clone. If a rapidresponse reporter, such as Rex or Nanog promoter fused to a reportergene, was employed it would be possible to plate cells at a low densityin semi-solid media, harvest and then screen by flow cytometry.Alternatively, the stop codon downstream of the antibody gene in pINT19could be replaced by a transmembrane domain to enable tethering of theantibody to the cell surface. The stop codon downstream of the antibodygene in pINT19 could also be replaced by an endoplasmic reticulum (ER)retention signal sequence to enable retention of antibodies in the ERand potential down-regulation of an endogenously expressed targetreceptor or any secreted protein or peptide. pINT19 is designedspecifically to target the mouse ROSA26 locus and can be employed forphenotypic screening of antibodies or alternative binding molecules inmouse ES cells. However, nuclease-directed antibody or binder moleculegene integration methods could also be applied to other functionalscreens such as those described using the lentiviral approach[47,106,107,108].

REFERENCES

All references listed below and others cited anywhere in this disclosureare incorporated herein by reference in their entirety.

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What is claimed is:
 1. A method of producing a library of eukaryoticcell clones containing DNA encoding a diverse repertoire of antibodymolecules which differ in one or more CDRs, comprising providing donorDNA molecules encoding antibody fragments comprising one or more CDRs,and higher eukaryotic cells with a genome size of greater than 2×10⁷base pairs, introducing the donor DNA into the cells and providing asite-specific nuclease within the cells, wherein the nuclease cleaves arecognition sequence in cellular DNA to create an integration site atwhich the donor DNA becomes integrated into the cellular DNA,integration occurring through DNA repair mechanisms endogenous to thecells, thereby creating recombinant cells containing donor DNAintegrated in the cellular DNA, and culturing the recombinant cells toproduce clones, thereby providing a library of eukaryotic cell clonescontaining donor DNA encoding the repertoire of antibody molecules. 2.The method of claim 1, wherein the antibody molecules have sequencediversity in heavy chain CDR3.
 3. The method of claim 1, wherein theantibody molecules have sequence diversity in all six CDRs.
 4. Themethod of claim 1, wherein the donor DNA is linear DNA.
 5. The method ofclaim 1, wherein the donor DNA is single stranded DNA.
 6. The method ofclaim 4, wherein the donor DNA is linearised double-stranded plasmid DNAor PCR product or synthetic genes.
 7. The method of claim 1, wherein thecells are mammalian cells.
 8. The method of claim 7, wherein the cellsare B lymphocyte lineage cells.
 9. The method of claim 8, wherein thecells are B cells or a B cell line.
 10. The method of claim 1, whereinthe antibody molecules are IgG, Fab, scFv-Fc or scFv, wherein theantibody molecules differ in one or more CDRs.
 11. The method of claim1, wherein the antibody molecules are multimeric, comprising at least afirst and a second subunit.
 12. The method of claim 1, wherein therecognition sequence is in genomic DNA of the cells, wherein the cellscontain endogenous sequences recognised by the site-specific nuclease orwherein the recognition sequence is engineered into the cellular DNA.13. The method of claim 1, wherein the site-specific nuclease cleavescellular DNA to create a double strand break serving as an integrationsite.
 14. The method of claim 1, wherein the nuclease is a meganuclease,a zinc finger nuclease (ZFN), a TALE nuclease or a nucleic acid guidednuclease.
 15. The method of claim 14, wherein DNA cleavage is directedby the CRISPR/Cas system.
 16. The method of claim 1, wherein the librarycontains at least 100, 10³ or 10⁴ clones, each clone being derived froman individual recombinant cell produced by integration of donor DNA. 17.The method of claim 16, wherein the library contains at least 10⁵ or 10⁶clones.
 18. A method of screening for a binder that recognises a target,comprising: producing a library by the method of claim 1, culturingcells of the library to express the binders, exposing the binders to thetarget, allowing recognition of the target by one or more cognatebinders, if present, and detecting whether the target is recognised by acognate binder.
 19. The method of claim 18, further comprising detectingtarget recognition by a cognate binder, and recovering cells of a clonecontaining DNA encoding the cognate binder.
 20. The method of claim 19,further comprising isolating nucleic acid encoding the binder from therecovered clone, thereby obtaining nucleic acid encoding a binder thatrecognises the target.
 21. The method of claim 20, comprising optionallyintroducing mutation or converting the nucleic acid to modified nucleicacid encoding a restructured binder, and introducing DNA encoding thebinder into a host cell.
 22. The method of claim 21, further comprisingculturing the cells to express the binder, and purifying the binder. 23.A method of screening for a cell of a desired phenotype, wherein thephenotype results from expression of a binder by the cell, the methodcomprising producing a library by the method of claim 1, culturing thelibrary cells to express the binders, and detecting whether the desiredphenotype is exhibited.
 24. The method of claim 23, further comprisingrecovering cells of a clone that expresses a binder that produces thedesired phenotype.
 25. The method of claim 24, further comprisingisolating nucleic acid encoding the binder from the recovered clone,thereby obtaining nucleic acid encoding a binder which produces thedesired phenotype.
 26. The method of claim 25, comprising optionallyintroducing mutation or converting the nucleic acid to modified nucleicacid encoding a restructured binder, and introducing DNA encoding thebinder into a host cell.
 27. The method of claim 26, further comprisingculturing the cells to express the binder, and purifying the binder.